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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
<HTML>
<HEAD>
<title>SGI Proposal for F-3: Interface Section</title>
<link rel=stylesheet href=small-table.css type="text/css">
<link rel=stylesheet href=code.css type="text/css">
<hr>
<font size=6><i><b>
<p>
SGI Proposal for F-3: Interface Section
</b></i></font size>
<font size=-1>
<p>
<i>Last updated 2 May 2000</i>
</center>
</HEAD>
<BODY>
<p>
<hr>
<p>
<H3> <A NAME="Intro"></A>Introduction</H3>
<p>
The purpose of this proposal is to address Issue F-3, consistency checks,
along with F-4, empty throw specs,
and F-10, function return types.
<p>
The proposal is based on a facility that has been in use on SGI/MIPS
systems for several years,
with the objective of catching mismatches between function calls and
definitions.
In the form implemented,
it is capable of catching result and parameter type mismatches
(not precisely in the case of structured types),
and specifically of catching problems resulting from passing floating
point parameters in floating point registers to varargs functions
expecting them in integer registers (with much less overhead).
<p>
It has been modified below (in purple) to:
<ul>
<li> catch throw specification mismatches,
<li> catch template specialization/instantiation mismatches, and
<li> catch precise structured type mismatches.
</ul>
<p>
It is worth noting that this facility is quite simple in concept,
and its implementation at SGI required little effort.
Nevertheless, the description is a bit involved,
due to two sources of complexity:
<ul>
<li>
We have attempted to minimize the descriptor size,
since there may potentially be a large number of them.
Thus, we have many optional fields present only if required.
<p>
<li>
We support an even smaller descriptor which is sufficient to catch
floating point varargs parameter problems,
since old non-ANSI-compliant code may have them (on MIPS and IA-64),
without full-blown parameter profiles.
</ul>
<p>
<hr>
<p>
<H2> <A NAME="InterfaceSection"></A>Interface Section</H2>
<p>
Interface correctness checking by the linkers
(<b><i>ld</b></i>/<b><i>ld.so</b></i>),
as well as object file transformations,
require information about subprogram interfaces,
especially parameter profiles.
This section is intended to provide this information.
There should be one such section per object file
(including executables and DSOs).
<p>
These descriptors shall be used to describe both actual subprogram
definitions and the parameter profiles of calls.
In the latter case, various information will be missing,
and once the call is verified to match the profile of a callee,
references to its descriptor (e.g. from the events section)
may usually be converted to references to the callee's definition descriptor,
and the call descriptor may be removed.
<p>
The information to be provided has variable length.
Thus, the section's
contents are organized as a sequence of variable-length descriptors,
each with a fixed-length header possibly followed by variable-length data.
The descriptors must be sorted by the symbol table index in field symbol.
Each descriptor must be a multiple of 8 bytes in size,
with null padding between descriptors as required.
<p>
Its section attributes are:
<p>
<TABLE align=center BORDER=2 CELLSPACING=2 CELLPADDING=2>
<TR class=tiny>
<TD><b> name </b></TD>
<TD> <code> .IA_64.interfaces</code> </TD>
</TR>
<TR class=tiny>
<TD><code> sh_type </code></TD>
<TD><code> SHT_IA_64_IFACE </code> </TD>
</TR>
<TR class=tiny>
<TD><code> sh_link </code></TD>
<TD> Section header index of the associated symbol table </TD>
</TR>
<TR class=tiny>
<TD><code> sh_info </code></TD>
<TD> 0 </TD>
</TR>
<TR class=tiny>
<TD><code> sh_flags </code></TD>
<TD><code> SHF_SGI_NOSTRIP </code></TD>
</TR>
<TR class=tiny>
<TD> requirements </TD>
<TD> Must not be stripped </TD>
</TR>
</TABLE>
<p>
The structure of an interface descriptor fixed-length header is given
by the figure below.
<p>
<TABLE align=center BORDER=2 CELLSPACING=2 CELLPADDING=2>
<hr width=100%>
<CAPTION>
<B>Figure 4-30: Interface Descriptor Header Format</B>
</CAPTION>
<TR class=small>
<th align=left> <B> Field Name </B> </th>
<th align=center> <B> Type </B> </th>
<th align=left> <B> Comments </B> </th>
</TR>
<TR class=small>
<TD><tt><i> symbol </i></tt></TD>
<TD><tt> ElfXX_Word </tt></TD>
<TD> Symbol table index of subprogram name, or 0 for an indirect call</TD>
</TR>
<TR class=small>
<TD><tt><i> attrs </i></tt></TD>
<TD><tt> ElfXX_Half </tt></TD>
<TD> Attributes: see Figure 4-31 </TD>
</TR>
<TR class=small>
<TD><tt><i> pcnt </i></tt></TD>
<TD><tt> ElfXX_Byte </tt></TD>
<TD> Parameter count <sup>a, b</sup></TD>
</TR>
<TR class=small>
<TD><tt><i> fpmask </i></tt></TD>
<TD><tt> ElfXX_Byte </tt></TD>
<TD> Mask of FP parameter registers <sup>c</sup></TD>
</TR>
</TABLE>
<p>
Notes:
<dl>
<dt> <tt><i> pcnt </i></tt> </dt>
<dd>
<sup>a</sup>
The parameter count and parameter profile below describe the parameter
profile as transformed (not as declared).
They include implicit parameters inserted by the compiler,
including function results converted to implicit result pointers
passed as parameters.
<p>
<sup>b</sup>
The parameter count includes the result if the
<b>SA_FUNCTION</b> attribute is set,
and the first descriptor is for the result.
If the parameter count is 255,
then the parameter list is preceded by a two-byte parameter count.
See Figure 4-32.)
</dd>
<p>
<dt> <tt><i> fpmask </i></tt> </dt>
<dd>
<sup>c</sup>
This mask indicates which of the eight FP parameter registers are used
to pass parameters instead of the corresponding integer registers.
The function result is not considered if <b>SA_FUNCTION</b> is set.
The lowest-order bits of the mask represent the first parameters,
i.e. 0x01 is parameter #1 in f8, 0x02 parameter #2 in f9, etc.
</dd>
</dl>
<p><hr>
<p>
The attributes of a subprogram are encoded in the <i>attrs</i> field as
the following bits.
<p>
<TABLE align=center BORDER=2 CELLSPACING=2 CELLPADDING=2>
<hr width=100%>
<CAPTION>
<B>Figure 4-31: Interface Descriptor Subprogram Attributes</B>
</CAPTION>
<TR>
<th align=left> <B> Attribute Name </B> </th>
<th align=center> <B> Value </B> </th>
<th align=left> <B> Comments </B> </th>
</TR>
<TR>
<TD><b> SA_PROTOTYPED </b></TD>
<TD><tt> 0x8000 </tt></TD>
<TD> Does def or ref have prototype?</TD>
</TR>
<TR>
<TD><b> SA_VARARGS </b></TD>
<TD><tt> 0x4000 </tt></TD>
<TD> Is this a varargs subprogram?</TD>
</TR>
<TR>
<TD><b> <font color=purple> SA_INSTANTIATION </font></b></TD>
<TD><tt> 0x2000 </tt></TD>
<TD> Is this a template instantiation?</TD>
</TR>
<TR>
<TD><b> <font color=purple> SA_SPECIALIZATION </font></b></TD>
<TD><tt> 0x1000 </tt></TD>
<TD> Is this a template specialization?</TD>
</TR>
<TR>
<TD><b> SA_FUNCTION </b></TD>
<TD><tt> 0x0400 </tt></TD>
<TD> Does subprogram return a result? </TD>
</TR>
<TR>
<TD><b> SA_NESTED </b></TD>
<TD><tt> 0x0200 </tt></TD>
<TD> Is subprogram nested? </td>
</TR>
<TR>
<TD><b> SA_IGNORE_ERROR </b></TD>
<TD><tt> 0x0100 </tt></TD>
<TD> Don't enforce consistency. </td>
</TR>
<TR>
<TD><b> SA_DEFINITION </b></TD>
<TD><tt> 0x0080 </tt></TD>
<TD> Is this a definition (not just call)?</TD>
</TR>
<TR>
<TD><b> <font color=purple> SA_THROW_SPEC </font></b></TD>
<TD><tt> 0x0040 </tt></TD>
<TD> C++ throw specification follows</TD>
</TR>
<TR>
<TD><b> SA_FREE_REGS </b></TD>
<TD><tt> 0x0020 </tt></TD>
<TD> Free register mask follows. </td>
</TR>
<TR>
<TD><b> SA_PARAMETERS </b></TD>
<TD><tt> 0x0010 </tt></TD>
<TD> Parameter profile follows. </td>
</TR>
</TABLE>
<p>
Notes:
<dl>
<p>
<font color=purple>
<dt> SA_INSTANTIATION </dt>
<dd>
This function was produced as the result of C++ template instantiation.
If it matches another interface descriptor without this flag set,
there is a conflict between an instantiation and a specialization.
</dd>
<p>
<dt> SA_SPECIALIZATION </dt>
<dd>
This function was produced as the result of C++ template specialization,
full or partial.
If it matches another interface descriptor without this flag set,
there is a conflict between an instantiation and a specialization.
</dd>
</font>
<p>
<dt> SA_FUNCTION </dt>
<dd>
This is specified as transformed by the compiler, not necessarily as declared,
e.g. a subprogram treating a structure result by placing it in a buffer
addressed as an implicit first parameter would not be encoded as a function.
</dd>
<p>
<dt> SA_NESTED </dt>
<dd>
Because nested subprograms require a static link in addition to the
usual declared parameters,
if a definition has this attribute all calls should too,
but the opposite condition is not necessary.
</dd>
<p>
<dt> SA_IGNORE_ERROR </dt>
<dd>
Some subprograms may be known to be called inconsistently.
This attribute indicates that any tools checking for inconsistencies
should not reject the objects due to inconsistencies for this subprogram.
</dd>
<font color=purple>
<p>
<dt> SA_THROW_SPEC </dt>
<dd>
The parameter profile is followed by a C++ throw specification.
</dd>
</font>
<p>
<dt> SA_FREE_REGS </dt>
<dd>
A 32-bit free register mask precedes the parameter profile,
specifying integer global registers which are never used in this routine.
A program transformation tool like pixie may use these registers,
subject to the ABI assumptions about caller-saved registers.
The mask must be updated by such a tool if registers are used.
See Figure 4-32.)
</dd>
<p>
<dt> SA_PARAMETERS </dt>
<dd>
Minimal checking may be achieved by including only the fixed-length
part of the interface descriptor,
and omitting the detailed parameter profile.
Doing so changes the meaning of the <i>pcnt</i> field
in the fixed-length header.
See the description of required linker checking below.
</dd>
</dl>
<p><hr>
<p>
The variable-length part of an interface descriptor consists of a list
of parameter descriptors following the fixed-length header,
possibly preceded by a profile size, full parameter count,
free register mask,
<font color=purple>
and/or throw specification offset and count.
</font>
It contains one descriptor for each parameter
(formal for definitions, actual for calls),
plus a leading descriptor for the result of functions.
Ellipsis parameters are not present for varargs subprograms;
the <b>SA_VARARGS</b> attribute indicates this case.
After resolving a reference in a call,
the compiler or linker should replace the caller's actual interface
reference by a reference to the callee's formal interface if they match.
(Observe that a varargs call will never match unless no variable
parameters are passed.
Therefore, the call descriptor should not be removed unless the callee
is defined and non-preemptible,
though it may be merged with others that have the same profile.)
<p>
The structure of the variable-length part of an interface descriptor
is a parameter profile with the form given in the figure below.
<p>
<TABLE align=center BORDER=2 CELLSPACING=2 CELLPADDING=2>
<hr width=100%>
<CAPTION>
<B>Figure 4-32: Interface Descriptor Parameter Profile</B>
</CAPTION>
<TR>
<th align=center> <B> Size </B> </th>
<th align=left> <B> Description </B> </th>
</TR>
<TR>
<TD> 2 bytes </TD>
<TD> Size in bytes of profile if any other fields are present </TD>
</TR>
<TR>
<TD> 2 bytes </TD>
<TD> Parameter count if <i>pcnt</i> == 255, else 0 </TD>
</TR>
<TR>
<TD> 4 bytes </TD>
<TD> Free register mask if <b>SA_FREE_REGS</b> is set </TD>
</TR>
<TR>
<TD> 2 bytes </TD>
<TD> <font color=purple>
Offset (from size field) of throw spec if SA_THROW_SPEC is set
</font> </TD>
</TR>
<TR>
<TD> 2 bytes </TD>
<TD> <font color=purple>
Number of throw spec entries if SA_THROW_SPEC is set
</font> </TD>
</TR>
<TR>
<TD> 2+ bytes </TD>
<TD> Result type descriptor if <b>SA_FUNCTION</b> is set </TD>
</TR>
<TR>
<TD> 2+ bytes </TD>
<TD> Parameter #1 type descriptor </TD>
</TR>
<TR>
<TD> 2+ bytes </TD>
<TD> Parameter #2 type descriptor </TD>
</TR>
<TR>
<TD> 2+ bytes </TD>
<TD> ... </TD>
</TR>
<TR>
<TD> 2+ bytes </TD>
<TD> Parameter #<i>pcnt</i> type descriptor </TD>
</TR>
<TR>
<TD> 4 bytes </TD>
<TD> <font color=purple> First throw spec identifier (aligned) </font> </TD>
</TR>
<TR>
<TD> 4 bytes </TD>
<TD> <font color=purple> Second throw spec identifier </font> </TD>
</TR>
<TR>
<TD> </TD>
<TD> ... </TD>
</TR>
<TR>
<TD> 4 bytes </TD>
<TD> <font color=purple> Last throw spec identifier </font> </TD>
</TR>
</TABLE>
<p><hr>
<p>
The structure of each type descriptor is given in the figure below.
<p>
<TABLE align=center BORDER=2 CELLSPACING=2 CELLPADDING=2>
<hr width=100%>
<CAPTION>
<B>Figure 4-33: Interface Descriptor Parameter Type Descriptor</B>
</CAPTION>
<TR>
<th align=center> <B> Byte # </B> </th>
<th align=left> <B> Description </B> </th>
</TR>
<TR>
<TD> 1 </TD>
<TD> Flags (high-order nibble) and qualifier count (low-order nibble) </TD>
</TR>
<TR>
<TD> 2 </TD>
<TD> Fundamental type (see Figure 4-35 below) </TD>
</TR>
<TR>
<TD> 3 or 3..6</TD>
<TD> Size if indeterminate for type,
<br>1 or 4 bytes (not aligned) depending on PDM_SIZE flag </TD>
</TR>
<TR>
<TD> j..j+3 (3,4,7) </TD>
<TD> <font color=purple>
RTTI symbol index (4 bytes, not aligned) if PDM_RTTI flag is set
</font> </TD>
</TR>
<TR>
<TD> k (>=3) </TD>
<TD> First qualifier </TD>
</TR>
<TR>
<TD> k+1 </TD>
<TD> Second qualifier </TD>
</TR>
<TR>
<TD> k+2 </TD>
<TD> Additional qualifiers </TD>
</TR>
</TABLE>
<p><hr>
<p>
Each parameter type descriptor begins with two bytes.
It contains several flags and a qualifier count in the first byte
as described in Figure 4-34 below,
and one of the fundamental types from Figure 4-35 below
in the second byte.
Next comes the actual parameter length (in bytes)
for those types with indeterminate length (e.g. FT_struct, FT_union).
If PDM_SIZE is not set, the length is an unsigned byte,
where the maximum value (255) implies at least that length;
if PDM_SIZE is set it is a 32-bit unsigned word (not necessarily aligned),
where the maximum value (0xffffffff) implies at least that length.
<font color=purple>
If PDM_RTTI is set,
the next 4-byte unaligned field is a symbol table index of the RTTI
descriptor for the type.
</font>
If the qualifier count is non-zero,
it will be followed by that number of single-byte type qualifiers
from Figure 4-36 below.
<p>
The masks for the initial byte of each type descriptor are
given in the figure below.
<p>
<TABLE align=center BORDER=2 CELLSPACING=2 CELLPADDING=2>
<hr width=100%>
<CAPTION>
<B>Figure 4-34: Interface Descriptor Parameter Type Masks</B>
</CAPTION>
<TR>
<th align=left> <B> Mask Name </B> </th>
<th align=center> <B> Value </B> </th>
<th align=left> <B> Comments </B> </th>
</TR>
<tr><td colspan=3></td></tr>
<TR>
<TD><b> PDM_SIZE </b></TD>
<TD align=center> 0x80 </TD>
<TD> Precise size present? </TD>
</TR>
<TR>
<TD><b> PDM_REFERENCE </b></TD>
<TD align=center> 0x40 </TD>
<TD> Reference parameter? </TD>
</TR>
<TR>
<TD><b> PDM_RTTI </b></TD>
<TD align=center> 0x20 </TD>
<TD> RTTI symbol for type? </TD>
</TR>
<TR>
<TD><b> PDM_QUALIFIERS </b></TD>
<TD align=center> 0x0f </TD>
<TD> Count of type qualifiers >>8 </TD>
</TR>
</TABLE>
<p><hr>
<p>
The possible fundamental types for a type descriptor
(adapted from, but not identical to, DWARF version 1)
are given in the figure below.
<p>
<TABLE align=center BORDER=2 CELLSPACING=2 CELLPADDING=2>
<hr width=100%>
<CAPTION>
<B>Figure 4-35: Fundamental Type Names</B>
</CAPTION>
<TR>
<th align=left> <B> Name </B> </th>
<th align=center> <B> Value </B> </th>
<th align=left> <B> Comments </B> </th>
</TR>
<tr><td colspan=3></td></tr>
<TR>
<TD><b> FT_unknown </b></TD>
<TD align=center> 0x00 </TD>
<TD> unknown type </TD>
</TR>
<TR>
<TD><b> FT_signed_char </b></TD>
<TD align=center> 0x01 </TD>
<TD> 8-bit signed character </TD>
</TR>
<TR>
<TD><b> FT_unsigned_char </b></TD>
<TD align=center> 0x02 </TD>
<TD> 8-bit unsigned character </TD>
</TR>
<TR>
<TD><b> FT_signed_short </b></TD>
<TD align=center> 0x03 </TD>
<TD> 16-bit signed short integer </TD>
</TR>
<TR>
<TD><b> FT_unsigned_short </b></TD>
<TD align=center> 0x04 </TD>
<TD> 16-bit unsigned short integer </TD>
</TR>
<TR>
<TD><b> FT_signed_int32 </b></TD>
<TD align=center> 0x05 </TD>
<TD> 32-bit signed integer </TD>
</TR>
<TR>
<TD><b> FT_unsigned_int32 </b></TD>
<TD align=center> 0x06 </TD>
<TD> 32-bit unsigned integer </TD>
</TR>
<TR>
<TD><b> FT_signed_int64 </b></TD>
<TD align=center> 0x07 </TD>
<TD> 64-bit signed integer </TD>
</TR>
<TR>
<TD><b> FT_unsigned_int64 </b></TD>
<TD align=center> 0x08 </TD>
<TD> 64-bit unsigned integer </TD>
</TR>
<TR>
<TD><b> FT_pointer32 </b></TD>
<TD align=center> 0x09 </TD>
<TD> 32-bit pointer </TD>
</TR>
<TR>
<TD><b> FT_pointer64 </b></TD>
<TD align=center> 0x0a </TD>
<TD> 64-bit pointer </TD>
</TR>
<TR>
<TD><b> FT_float32 </b></TD>
<TD align=center> 0x0b </TD>
<TD> 32-bit IEEE floating point </TD>
</TR>
<TR>
<TD><b> FT_float64 </b></TD>
<TD align=center> 0x0c </TD>
<TD> 64-bit IEEE floating point </TD>
</TR>
<TR>
<TD><b> FT_float128 </b></TD>
<TD align=center> 0x0d </TD>
<TD> 128-bit floating point </TD>
</TR>
<TR>
<TD><b> FT_complex64 </b></TD>
<TD align=center> 0x0e </TD>
<TD> 64-bit IEEE complex floating point </TD>
</TR>
<TR>
<TD><b> FT_complex128 </b></TD>
<TD align=center> 0x0f </TD>
<TD> 128-bit IEEE complex floating point </TD>
</TR>
<TR>
<TD><b> FT_complex256 </b></TD>
<TD align=center> 0x10 </TD>
<TD> 256-bit complex floating point </TD>
</TR>
<TR>
<TD><b> FT_void </b></TD>
<TD align=center> 0x11 </TD>
<TD> void type </TD>
</TR>
<TR>
<TD><b> FT_bool32 </b></TD>
<TD align=center> 0x12 </TD>
<TD> 32-bit Boolean (TRUE or FALSE) </TD>
</TR>
<TR>
<TD><b> FT_bool64 </b></TD>
<TD align=center> 0x13 </TD>
<TD> 64-bit Boolean (TRUE or FALSE) </TD>
</TR>
<TR>
<TD><b> FT_label32 </b></TD>
<TD align=center> 0x14 </TD>
<TD> 32-bit label (address) </TD>
</TR>
<TR>
<TD><b> FT_label64 </b></TD>
<TD align=center> 0x15 </TD>
<TD> 64-bit label (address) </TD>
</TR>
<TR>
<TD><b> FT_float80 </b></TD>
<TD align=center> 0x16 </TD>
<TD> 80-bit floating point </TD>
</TR>
<TR>
<TD><b> FT_complex160 </b></TD>
<TD align=center> 0x17 </TD>
<TD> 2x80-bit complex floating point </TD>
</TR>
<tr><td colspan=3></td></tr>
<TR>
<TD><b> FT_struct </b></TD>
<TD align=center> 0x20 </TD>
<TD> structure (record) </TD>
</TR>
<TR>
<TD><b> FT_union </b></TD>
<TD align=center> 0x21 </TD>
<TD> union (variant) </TD>
</TR>
<TR>
<TD><b> FT_enum </b></TD>
<TD align=center> 0x22 </TD>
<TD> enumerated type </TD>
</TR>
<TR>
<TD><b> FT_class </b></TD>
<TD align=center> 0x28 </TD>
<TD> C++ class </TD>
</TR>
</TABLE>
<p><hr>
<p>
The fundamental types in Figure 4-35 may be modified by the qualifiers in
the figure below, also based on [DWARF-1]:
<p>
<TABLE align=center BORDER=2 CELLSPACING=2 CELLPADDING=2>
<hr width=100%>
<CAPTION>
<B>Figure 4-36: Type Qualifiers </B>
</CAPTION>
<TR>
<th align=left> <B> Name </B> </th>
<th align=center> <B> Value </B> </th>
<th align=left> <B> Comments </B> </th>
</TR>
<tr><td colspan=3></td></tr>
<TR>
<TD><b> MOD_pointer_to </b></TD>
<TD align=center> 0x01 </TD>
<TD> Pointer to base type </TD>
</TR>
<TR>
<TD><b> MOD_reference_to </b></TD>
<TD align=center> 0x02 </TD>
<TD> C++ reference to base type </TD>
</TR>
<TR>
<TD><b> MOD_const </b></TD>
<TD align=center> 0x03 </TD>
<TD> const base type </TD>
</TR>
<TR>
<TD><b> MOD_volatile </b></TD>
<TD align=center> 0x04 </TD>
<TD> volatile base type </TD>
</TR>
<TR>
<TD><b> MOD_function </b></TD>
<TD align=center> 0x05 </TD>
<TD> Function returning base type </TD>
</TR>
<TR>
<TD><b> MOD_array_of </b></TD>
<TD align=center> 0x06 </TD>
<TD> Array of base type </TD>
</TR>
</TABLE>
<p><hr>
<p>
<font color=purple>
If SA_THROW_SPEC is set,
the parameter profiles are followed by an array of identifiers
for the types that may be thrown by this routine (in C++).
The offset field gives the offset from the size field of the first
such identifier, at 4-byte alignment (with padding if necessary).
The count field gives the number of identifiers present.
Each identifier is the 4-byte symbol table index of an RTTI structure
for the type.
If the function has an empty throw specification,
offset and count should both be zero.
If SA_THROW_SPEC is not set,
nothing may be assumed about the throw specification;
it may not be checked.
</font>
<p>
<H3><A NAME="OptionalFields"></A>Optional Fields</H3>
<p>
To allow minimal overhead checking of varargs floating point parameters,
as well as full parameter consistency checking,
various parts of the parameter profile, as well as the whole profile,
are optional.
The following rules clarify which parts may be omitted:
<ul>
<p>
<li>
The entire profile is omitted if <b>SA_PARAMETERS</b>,
<b>SA_FREE_REGS</b>, and
<font color=purple> <b>SA_THROW_SPEC</b> </font>
are not set, and <i>pcnt</i> is not 255.
See the linker processing description of minimal checking below
for the treatment in this case.
<p>
<li>
If any profile component is present, the size field must be present.
<p>
<li>
If <i>pcnt</i> is 255, the parameter count field must be present,
equal to the number of parameters, including the result for functions.
(Note that this is the number of parameter register equivalents if
<b>SA_PARAMETERS</b> is not set --
see the linker processing description of minimal checking below.)
If <i>pcnt</i> is smaller, but <b>SA_FREE_REGS</b> is set,
the parameter count field is present (containing zero) for alignment.
<p>
<li>
If <b>SA_FREE_REGS</b> is set, the free register mask must be present
(as well as the parameter count field).
<p>
<li>
<font color=purple>
If <b>SA_THROW_SPEC</b> is set,
the throw specification offset and count must be present,
and the indicated number of type identifiers must follow the parameter
type descriptors.
</font>
<p>
<li>
If <b>SA_PARAMETERS</b> is set,
the number of parameter type descriptors implied by <i>pcnt</i>
or the parameter count field is present.
The first one is the function result type if <b>SA_FUNCTION</b> is set.
These descriptors are described in more detail below.
<p>
<li>
Finally, if the full profile is not a multiple of eight bytes long,
it is padded to a multiple of eight bytes with zeros
(which are not included in the size).
</ul>
<p>
<H3><A NAME="InterfaceLinking"></A>Linker Processing</H3>
<i>
<p>
Full support of the interface descriptor section requires significant
processing by the linker -- its purpose is to provide link-time checking.
This processing involves checking and compression.
This facility has been designed to be used either for minimal checking
with minimal space requirements, or for full checking.
<p>
<h4>Full checking</h4>
<p>
The compiler may generate a descriptor for every subprogram definition
and potentially for every call.
(Only one descriptor is required for multiple calls to the same subprogram,
and if the callee is in the same compilation,
the compiler can check parameters and omit the call descriptors entirely.)
When the linker resolves a call to the callee's definition,
it should check the call descriptor against the definition descriptor.
The rules for compatibility are as follows:
<ol>
<p>
<li>
If this is not a varargs routine (not SA_VARARGS),
then the number of parameters should match.
For each parameter the sizes should match,
and whether it is integer or floating point should match.
<p>
<li>
For a varargs routine (SA_VARARGS),
the fixed parameters should match
(except perhaps for the last one,
which may be a varargs.h va_alist dummy parameter),
and there must be no float ing point parameters in the variable part
unless the call site had a prototype visible (SA_PROTOTYPED).
</ol>