tor-browser

commit f6e86e9a5e2b3a964e777495238bced8cb2126e8
parent c2821adfbab154bfd4eb3a4bd8bed2c0d6d50ff7
Author: André Bargull <andre.bargull@gmail.com>
Date:   Thu, 23 Oct 2025 09:56:06 +0000

Bug 1995488 - Part 1: Avoid comparison against non-zero in clampIntToUint8 for riscv64. r=spidermonkey-reviewers,iain

`ClampIToUint8` was compiled to:
```asm
slti      t4, t0, 0
li        t5, 1
bne       t4, t5, 0 -> skip
mv        t0, zero
j         0 -> skip2
li        t5, 255
bge       t5, t0, 0 -> skip2
li        t0, 255
```

Loading the immediate `1` into `t5` is unnecessary when instead comparing
against zero. But we can go even further and use the same approach as used
in `MacroAssemblerRiscv64Compat::minMax32` to check for negative values. That
saves another branch and `ClampIToUint8` is now compiled to:
```asm
sgtz      t4, t0
neg       t4, t4
and       t0, t0, t4
li        t4, 255
bge       t4, t0, 0 -> skip
li        t0, 255
```

Differential Revision: https://phabricator.services.mozilla.com/D269350

Diffstat:
M
js/src/jit/riscv64/MacroAssembler-riscv64-inl.h
 | 
27
++++++++++++++++-----------

1 file changed, 16 insertions(+), 11 deletions(-)
diff --git a/js/src/jit/riscv64/MacroAssembler-riscv64-inl.h b/js/src/jit/riscv64/MacroAssembler-riscv64-inl.h
@@ -1324,18 +1324,23 @@ void MacroAssembler::byteSwap64(Register64 src) {
 }
 void MacroAssembler::clampIntToUint8(Register reg) {
   // If reg is < 0, then we want to clamp to 0.
-  Label skip, skip2;
-  UseScratchRegisterScope temps(this);
-  Register scratch = temps.Acquire();
-  slti(scratch, reg, 0);
-  ma_branch(&skip, NotEqual, scratch, Operand(1));
-  ma_li(reg, Imm32(0));
-  jump(&skip2);
-  bind(&skip);
+  {
+    UseScratchRegisterScope temps(this);
+    Register scratch = temps.Acquire();
+
+    // reg = -(reg > 0 ? 1 : 0) & reg
+    sgtz(scratch, reg);
+    neg(scratch, scratch);
+    and_(reg, reg, scratch);
+  }
+
   // If reg is >= 255, then we want to clamp to 255.
-  ma_branch(&skip2, LessThanOrEqual, reg, Operand(255));
-  ma_li(reg, Imm32(255));
-  bind(&skip2);
+  Label skip;
+  ma_branch(&skip, LessThanOrEqual, reg, Operand(255));
+  {
+    ma_li(reg, Imm32(255));
+  }
+  bind(&skip);
 }
 
 void MacroAssembler::clz32(Register src, Register dest, bool knownNotZero) {