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Document branch relaxation #58

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Document branch relaxation #58

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30 changes: 30 additions & 0 deletions riscv-asm.md
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Expand Up @@ -449,6 +449,36 @@ as seen by `objdump`:
14: 00028067 jr t0 # 0x10
```

Branches
--------------------

Unconditional branches are implemented by the `j(al)?r?` pseudoinstructions.
(The underlying instructions are `jalr?`.)
The `j(al)?` targets can be any symbol or address.
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Can it really be any symbol/address? I haven't tried them all.

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“Any” is indeed an overstatement, since the call/tail macros have a maximum displacement of roughly 2 GB.

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Thanks. I'll read up on this and improve it.


Conditional branches are implemented by the `b(l|g)(t|e)(z|u)?` and `b(eq|ne)z?` pseudoinstructions.
(The underlying instructions are `b(lt|ge)u?` and `b(eq|ne)`.)
Again, the targets can be any symbol or address.
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(Same as above.)

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Same here, except there’s a 1 MiB limit.


Various relaxations are performed when the target's offset from the branch exceeds the range of the underlying instruction's immediate field.

For example,

```assembly
beqz t0, foo
```

may be relaxed to a sequence of the form

```assembly
bnez t0, 1f
j foo
1:
```

The `bnez` is further relaxed to `bne`, while `j` is relaxed to `jal` with a relocation.
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These are not relaxations. These are pure aliases for specific forms (bne with a zero register, and jal with a zero register).



Floating-point rounding modes
-----------------------------

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