tor-browser

commit bfc5055cb73637fbae68ea69cd8d61edf26115c6
parent 897281b10610d7f70362971d8e1ddeb658e5ae9a
Author: André Bargull <andre.bargull@gmail.com>
Date:   Mon, 27 Oct 2025 15:22:14 +0000

Bug 1996346 - Part 2: Relax register constraints for UrshD on x86-shared. r=spidermonkey-reviewers,iain

We don't need `tempCopy(lhs)` when BMI2 is available. This can save an
unnecessary move.

For example:
```js
// Call with x=-1 and y=0.
function f(x, y) {
  return x >>> y;
}
```

was compiled to:
```asm
[Codegen]                                 # LIR=MoveGroup
[Codegen] movl       %eax, %edx
[Codegen]                                 # LIR=UrshD
[Codegen] shrxl      %edx, %ecx, %edx
[Codegen] xorpd      %xmm0, %xmm0
[Codegen] cvtsi2sd   %rdx, %xmm0
```

And is now compiled to:
```asm
[Codegen]                                 # LIR=UrshD
[Codegen] shrxl      %eax, %edx, %ecx
[Codegen] xorpd      %xmm0, %xmm0
[Codegen] cvtsi2sd   %rcx, %xmm0
```

Differential Revision: https://phabricator.services.mozilla.com/D270026

Diffstat:
M
js/src/jit/x86-shared/CodeGenerator-x86-shared.cpp
 | 
15
+++++++++++----
M
js/src/jit/x86-shared/Lowering-x86-shared.cpp
 | 
12
+++++++-----

2 files changed, 18 insertions(+), 9 deletions(-)
diff --git a/js/src/jit/x86-shared/CodeGenerator-x86-shared.cpp b/js/src/jit/x86-shared/CodeGenerator-x86-shared.cpp
@@ -1645,22 +1645,29 @@ void CodeGenerator::visitShiftI64(LShiftI64* lir) {
 
 void CodeGenerator::visitUrshD(LUrshD* ins) {
   Register lhs = ToRegister(ins->lhs());
-  MOZ_ASSERT(ToRegister(ins->temp0()) == lhs);
-
   const LAllocation* rhs = ins->rhs();
   FloatRegister out = ToFloatRegister(ins->output());
+  Register temp = ToRegister(ins->temp0());
 
   if (rhs->isConstant()) {
+    MOZ_ASSERT(temp == lhs);
+
     int32_t shift = ToInt32(rhs) & 0x1F;
     if (shift) {
       masm.shrl(Imm32(shift), lhs);
     }
   } else {
+    MOZ_ASSERT_IF(temp != lhs, Assembler::HasBMI2());
+
     Register shift = ToRegister(rhs);
-    masm.rshift32(shift, lhs);
+    if (temp != lhs) {
+      masm.shrxl(lhs, shift, temp);
+    } else {
+      masm.rshift32(shift, lhs);
+    }
   }
 
-  masm.convertUInt32ToDouble(lhs, out);
+  masm.convertUInt32ToDouble(temp, out);
 }
 
 Operand CodeGeneratorX86Shared::ToOperand(const LAllocation& a) {
diff --git a/js/src/jit/x86-shared/Lowering-x86-shared.cpp b/js/src/jit/x86-shared/Lowering-x86-shared.cpp
@@ -478,19 +478,21 @@ void LIRGeneratorX86Shared::lowerUrshD(MUrsh* mir) {
   MOZ_ASSERT(rhs->type() == MIRType::Int32);
   MOZ_ASSERT(mir->type() == MIRType::Double);
 
-#ifdef JS_CODEGEN_X64
-  static_assert(ecx == rcx);
-#endif
-
   LUse lhsUse = useRegisterAtStart(lhs);
   LAllocation rhsAlloc;
+  LDefinition tempDef;
   if (rhs->isConstant()) {
     rhsAlloc = useOrConstant(rhs);
+    tempDef = tempCopy(lhs, 0);
+  } else if (Assembler::HasBMI2()) {
+    rhsAlloc = useRegisterAtStart(rhs);
+    tempDef = temp();
   } else {
     rhsAlloc = useShiftRegister(rhs);
+    tempDef = tempCopy(lhs, 0);
   }
 
-  LUrshD* lir = new (alloc()) LUrshD(lhsUse, rhsAlloc, tempCopy(lhs, 0));
+  auto* lir = new (alloc()) LUrshD(lhsUse, rhsAlloc, tempDef);
   define(lir, mir);
 }