tor-browser

commit 881b2dd3dc1f85f7a5f20cdea442dcb3271d35e0
parent ba6a42666693c6db3626bd8b098419826f1057b8
Author: André Bargull <andre.bargull@gmail.com>
Date:   Tue, 11 Nov 2025 12:29:35 +0000

Bug 1998161 - Part 4: Merge shifts for division by constants for x64. r=iain

Otherwise we can end up performing two consecutive right-shifts.

Also don't emit shifts with a zero shift amount.

Differential Revision: https://phabricator.services.mozilla.com/D271432

Diffstat:
M
js/src/jit/x86-shared/CodeGenerator-x86-shared.cpp
 | 
32
+++++++++++++++++++++++++++-----

1 file changed, 27 insertions(+), 5 deletions(-)
diff --git a/js/src/jit/x86-shared/CodeGenerator-x86-shared.cpp b/js/src/jit/x86-shared/CodeGenerator-x86-shared.cpp
@@ -1020,7 +1020,9 @@ static void UnsignedDivideWithConstant(MacroAssembler& masm, LUDivOrUMod* ins,
     masm.movl(Imm32(rmc.multiplier), temp);
     masm.imulq(temp, result);
   }
-  masm.shrq(Imm32(32), result);
+  if (rmc.multiplier > UINT32_MAX || rmc.shiftAmount == 0) {
+    masm.shrq(Imm32(32), result);
+  }
 #endif
   if (rmc.multiplier > UINT32_MAX) {
     // M >= 2^32 and shift == 0 is impossible, as d >= 2 implies that
@@ -1044,9 +1046,17 @@ static void UnsignedDivideWithConstant(MacroAssembler& masm, LUDivOrUMod* ins,
 
     // Finish the computation.
     masm.addl(temp, result);
-    masm.shrl(Imm32(rmc.shiftAmount - 1), result);
+    if (rmc.shiftAmount > 1) {
+      masm.shrl(Imm32(rmc.shiftAmount - 1), result);
+    }
   } else {
-    masm.shrl(Imm32(rmc.shiftAmount), result);
+    if (rmc.shiftAmount > 0) {
+#ifdef JS_CODEGEN_X86
+      masm.shrl(Imm32(rmc.shiftAmount), result);
+#else
+      masm.shrq(Imm32(32 + rmc.shiftAmount), result);
+#endif
+    }
   }
 }
 
@@ -1236,7 +1246,9 @@ static void DivideWithConstant(MacroAssembler& masm, LDivOrMod* ins,
   // Sign-extend |lhs| in preparation for a 64-bit multiplication.
   masm.movslq(lhs, result);
   masm.imulq(Imm32(rmc.multiplier), result, result);
-  masm.shrq(Imm32(32), result);
+  if (rmc.multiplier > INT32_MAX || rmc.shiftAmount == 0) {
+    masm.shrq(Imm32(32), result);
+  }
 #endif
   if (rmc.multiplier > INT32_MAX) {
     MOZ_ASSERT(rmc.multiplier < (int64_t(1) << 32));
@@ -1250,7 +1262,17 @@ static void DivideWithConstant(MacroAssembler& masm, LDivOrMod* ins,
   // (M * n) >> (32 + shift) is the truncated division answer if n is
   // non-negative, as proved in the comments of computeDivisionConstants. We
   // must add 1 later if n is negative to get the right answer in all cases.
-  masm.sarl(Imm32(rmc.shiftAmount), result);
+  if (rmc.shiftAmount > 0) {
+#ifdef JS_CODEGEN_X86
+    masm.sarl(Imm32(rmc.shiftAmount), result);
+#else
+    if (rmc.multiplier > INT32_MAX) {
+      masm.sarl(Imm32(rmc.shiftAmount), result);
+    } else {
+      masm.sarq(Imm32(32 + rmc.shiftAmount), result);
+    }
+#endif
+  }
 
   // We'll subtract -1 instead of adding 1, because (n < 0 ? -1 : 0) can be
   // computed with just a sign-extending shift of 31 bits.