ISLE: aarch64: fix narrow sdiv on intmin/-1 (#9541)

* Fix a missing trap on narrow sdiv of intmin/-1 by left-shifting before checking for intmin Co-authored-by: Michael McLoughlin <[email protected]> * Remove non-upstreamed verifier attributes --------- Co-authored-by: Michael McLoughlin <[email protected]>
bytecodealliance · Nov 1, 2024 · 025fea0 · 025fea0
1 parent 1499840
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diff --git a/cranelift/codegen/src/isa/aarch64/inst.isle b/cranelift/codegen/src/isa/aarch64/inst.isle
@@ -1289,6 +1289,12 @@
             (Size64 64)
             (Size128 128)))
 
+;; Difference (32 - ty), useful for narrow calculations with 32-bit
+;; instructions.
+(decl diff_from_32 (Type) u8)
+(rule (diff_from_32 $I8) 24)
+(rule (diff_from_32 $I16) 16)
+
 (type ScalarSize extern
       (enum Size8
             Size16
@@ -3664,24 +3670,18 @@
 
 ;; Check for signed overflow. The only case is min_value / -1.
 ;; The following checks must be done in 32-bit or 64-bit, depending
-;; on the input type.
-(spec (trap_if_div_overflow ty x y)
-      (provide (= x result)
-      (if (= ty 32)
-          (not (and (= #x00000000 (extract 31 0 y))
-                 (= #x80000000 (extract 31 0 y))))
-          (not (and (= #x0000000000000000 y)
-                    (= #x8000000000000000 y))))))
-(decl trap_if_div_overflow (Type Reg Reg) Reg)
-(rule (trap_if_div_overflow ty x y)
+;; on the input type. For 8- and 16- bit, the check for x == min_value
+;; must use a possibly-shifted value, xcheck, to overflow as expected.
+(decl trap_if_div_overflow (Type Reg Reg Reg) Reg)
+(rule (trap_if_div_overflow ty xcheck x y)
       (let (
           ;; Check RHS is -1.
           (_ Unit (emit (MInst.AluRRImm12 (ALUOp.AddS) (operand_size ty) (writable_zero_reg) y (u8_into_imm12 1))))
 
-          ;; Check LHS is min_value, by subtracting 1 and branching if
-          ;; there is overflow.
+          ;; Check LHS is min_value, by subtracting 1 from the possibly-shifted
+          ;; value and branching if there is overflow.
           (_ Unit (emit (MInst.CCmpImm (size_from_ty ty)
-                                       x
+                                       xcheck
                                        (u8_into_uimm5 1)
                                        (nzcv $false $false $false $false)
                                        (Cond.Eq))))
@@ -3690,6 +3690,19 @@
         )
         x))
 
+;; In the cases narrower than a register width, subtracting 1 from the
+;; min_value will not cause overflow (e.g., I8's min_value of -128 stored in
+;; a 32-bit register produces -129 with no overflow). However, if we left shift
+;; x by (32 - ty), we then produce the 32-bit min_value for the respective min
+;; values of I8 and I16.
+;; E.g., I8's 0x00000080 left-shifted by 24 is 0x80000000, which overflows.
+(decl intmin_check (Type Reg) Reg)
+(rule intmin_check_fits_in_16 (intmin_check (fits_in_16 ty) x)
+      (alu_rr_imm_shift (ALUOp.Lsl) ty x (imm_shift_from_u8 (diff_from_32 ty))))
+
+;; In the I32 or I64 case, checking x itself against the min_value is fine.
+(rule -1 (intmin_check ty x) x)
+
 ;; Check for unsigned overflow.
 (decl trap_if_overflow (ProducesFlags TrapCode) Reg)
 (rule (trap_if_overflow producer tc)

diff --git a/cranelift/codegen/src/isa/aarch64/lower.isle b/cranelift/codegen/src/isa/aarch64/lower.isle
@@ -1055,7 +1055,7 @@
 ;; TODO: Add SDiv32 to implement 32-bit directly, rather
 ;; than extending the input.
 ;;
-;; The sequence of checks here should look like:
+;; The sequence of checks here should look like this for 32 or 64 bits:
 ;;
 ;;   cbnz rm, #8
 ;;   udf ; divide by zero
@@ -1064,6 +1064,9 @@
 ;;   b.vc #8
 ;;   udf ; signed overflow
 ;;
+;; In the narrow 8 or 16 bit case, we need to insert an additional left-shift
+;; to check for the minimum value using the 32-bit ccmp instruction.
+;;
 ;; Note The div instruction does not trap on divide by zero or overflow, so
 ;; checks need to be manually inserted.
 ;;
@@ -1072,7 +1075,8 @@
 (rule sdiv_base_case (lower (has_type (fits_in_64 ty) (sdiv x y)))
       (let ((x64 Reg (put_in_reg_sext64 x))
             (y64 Reg (put_nonzero_in_reg_sext64 y))
-            (valid_x64 Reg (trap_if_div_overflow ty x64 y64))
+            (intmin_check_x Reg (intmin_check ty x64))
+            (valid_x64 Reg (trap_if_div_overflow ty intmin_check_x x64 y64))
             (result Reg (a64_sdiv $I64 valid_x64 y64)))
         result))
 

diff --git a/cranelift/filetests/filetests/isa/aarch64/trap_sdiv.clif b/cranelift/filetests/filetests/isa/aarch64/trap_sdiv.clif
@@ -0,0 +1,123 @@
+test compile precise-output
+target aarch64
+
+function %div8(i8, i8) -> i8 {
+block0(v0: i8, v1: i8):
+  v2 = sdiv v0, v1
+  return v2
+}
+
+; VCode:
+; block0:
+;   sxtb x3, w0
+;   sxtb x5, w1
+;   cbz x5, #trap=int_divz
+;   lsl w8, w3, #24
+;   adds wzr, w5, #1
+;   ccmp w8, #1, #nzcv, eq
+;   b.vs #trap=int_ovf
+;   sdiv x0, x3, x5
+;   ret
+;
+; Disassembled:
+; block0: ; offset 0x0
+;   sxtb x3, w0
+;   sxtb x5, w1
+;   cbz x5, #0x24
+;   lsl w8, w3, #0x18
+;   cmn w5, #1
+;   ccmp w8, #1, #0, eq
+;   b.vs #0x28
+;   sdiv x0, x3, x5
+;   ret
+;   .byte 0x1f, 0xc1, 0x00, 0x00 ; trap: int_divz
+;   .byte 0x1f, 0xc1, 0x00, 0x00 ; trap: int_ovf
+
+function %div16(i16, i16) -> i16 {
+block0(v0: i16, v1: i16):
+  v2 = sdiv v0, v1
+  return v2
+}
+
+; VCode:
+; block0:
+;   sxth x3, w0
+;   sxth x5, w1
+;   cbz x5, #trap=int_divz
+;   lsl w8, w3, #16
+;   adds wzr, w5, #1
+;   ccmp w8, #1, #nzcv, eq
+;   b.vs #trap=int_ovf
+;   sdiv x0, x3, x5
+;   ret
+;
+; Disassembled:
+; block0: ; offset 0x0
+;   sxth x3, w0
+;   sxth x5, w1
+;   cbz x5, #0x24
+;   lsl w8, w3, #0x10
+;   cmn w5, #1
+;   ccmp w8, #1, #0, eq
+;   b.vs #0x28
+;   sdiv x0, x3, x5
+;   ret
+;   .byte 0x1f, 0xc1, 0x00, 0x00 ; trap: int_divz
+;   .byte 0x1f, 0xc1, 0x00, 0x00 ; trap: int_ovf
+
+function %div32(i32, i32) -> i32 {
+block0(v0: i32, v1: i32):
+  v2 = sdiv v0, v1
+  return v2
+}
+
+; VCode:
+; block0:
+;   sxtw x3, w0
+;   sxtw x5, w1
+;   cbz x5, #trap=int_divz
+;   adds wzr, w5, #1
+;   ccmp w3, #1, #nzcv, eq
+;   b.vs #trap=int_ovf
+;   sdiv x0, x3, x5
+;   ret
+;
+; Disassembled:
+; block0: ; offset 0x0
+;   sxtw x3, w0
+;   sxtw x5, w1
+;   cbz x5, #0x20
+;   cmn w5, #1
+;   ccmp w3, #1, #0, eq
+;   b.vs #0x24
+;   sdiv x0, x3, x5
+;   ret
+;   .byte 0x1f, 0xc1, 0x00, 0x00 ; trap: int_divz
+;   .byte 0x1f, 0xc1, 0x00, 0x00 ; trap: int_ovf
+
+function %div64(i64, i64) -> i64 {
+block0(v0: i64, v1: i64):
+  v2 = sdiv v0, v1
+  return v2
+}
+
+; VCode:
+; block0:
+;   cbz x1, #trap=int_divz
+;   adds xzr, x1, #1
+;   ccmp x0, #1, #nzcv, eq
+;   b.vs #trap=int_ovf
+;   sdiv x0, x0, x1
+;   ret
+;
+; Disassembled:
+; block0: ; offset 0x0
+;   cbz x1, #0x18
+;   cmn x1, #1
+;   ccmp x0, #1, #0, eq
+;   b.vs #0x1c
+;   sdiv x0, x0, x1
+;   ret
+;   .byte 0x1f, 0xc1, 0x00, 0x00 ; trap: int_divz
+;   .byte 0x1f, 0xc1, 0x00, 0x00 ; trap: int_ovf
+