tor-browser

commit 49a2579ade172287209d5ba5d477cbab9d1ae3ec
parent 402214953fc203f0bfcc27a4c9c2adaf552f70fd
Author: André Bargull <andre.bargull@gmail.com>
Date:   Thu, 23 Oct 2025 12:23:35 +0000

Bug 1995707 - Part 4: Do not bypass optimisations in Mov for registers. r=spidermonkey-reviewers,jandem

Backport of <https://github.com/Linaro/vixl/commit/1c78c34397f2c08c012733cc661076e8bd029eab>.

Differential Revision: https://phabricator.services.mozilla.com/D269554

Diffstat:
M
js/src/jit/arm64/MacroAssembler-arm64.cpp
 | 
5
++++-
M
js/src/jit/arm64/vixl/MacroAssembler-vixl.cpp
 | 
14
+-------------
M
js/src/jit/arm64/vixl/MacroAssembler-vixl.h
 | 
20
+++++++++++++++++---

3 files changed, 22 insertions(+), 17 deletions(-)
diff --git a/js/src/jit/arm64/MacroAssembler-arm64.cpp b/js/src/jit/arm64/MacroAssembler-arm64.cpp
@@ -3799,7 +3799,10 @@ void MacroAssembler::shiftIndex32AndAdd(Register indexTemp32, int shift,
       Operand(ARMRegister(indexTemp32, 64), vixl::LSL, shift));
 }
 
-void MacroAssembler::wasmMarkCallAsSlow() { Mov(x28, x28); }
+void MacroAssembler::wasmMarkCallAsSlow() {
+  // Use mov() instead of Mov() to ensure this no-op move isn't elided.
+  vixl::MacroAssembler::mov(x28, x28);
+}
 
 const int32_t SlowCallMarker = 0xaa1c03fc;
 
diff --git a/js/src/jit/arm64/vixl/MacroAssembler-vixl.cpp b/js/src/jit/arm64/vixl/MacroAssembler-vixl.cpp
@@ -480,19 +480,7 @@ void MacroAssembler::Mov(const Register& rd,
     EmitExtendShift(rd, operand.reg(), operand.extend(),
                     operand.shift_amount());
   } else {
-    // Otherwise, emit a register move only if the registers are distinct, or
-    // if they are not X registers.
-    //
-    // Note that mov(w0, w0) is not a no-op because it clears the top word of
-    // x0. A flag is provided (kDiscardForSameWReg) if a move between the same W
-    // registers is not required to clear the top word of the X register. In
-    // this case, the instruction is discarded.
-    //
-    // If the sp is an operand, add #0 is emitted, otherwise, orr #0.
-    if (!rd.Is(operand.reg()) || (rd.Is32Bits() &&
-                                  (discard_mode == kDontDiscardForSameWReg))) {
-      mov(rd, operand.reg());
-    }
+    Mov(rd, operand.reg(), discard_mode);
   }
 }
 
diff --git a/js/src/jit/arm64/vixl/MacroAssembler-vixl.h b/js/src/jit/arm64/vixl/MacroAssembler-vixl.h
@@ -1184,9 +1184,23 @@ class MacroAssembler : public js::jit::Assembler {
     SingleEmissionCheckScope guard(this);
     mneg(rd, rn, rm);
   }
-  void Mov(const Register& rd, const Register& rn) {
-    SingleEmissionCheckScope guard(this);
-    mov(rd, rn);
+  void Mov(const Register& rd,
+           const Register& rn,
+           DiscardMoveMode discard_mode = kDontDiscardForSameWReg) {
+    // Emit a register move only if the registers are distinct, or if they are
+    // not X registers.
+    //
+    // Note that mov(w0, w0) is not a no-op because it clears the top word of
+    // x0. A flag is provided (kDiscardForSameWReg) if a move between the same W
+    // registers is not required to clear the top word of the X register. In
+    // this case, the instruction is discarded.
+    //
+    // If the sp is an operand, add #0 is emitted, otherwise, orr #0.
+    if (!rd.Is(rn) ||
+        (rd.Is32Bits() && (discard_mode == kDontDiscardForSameWReg))) {
+      SingleEmissionCheckScope guard(this);
+      mov(rd, rn);
+    }
   }
   void Movk(const Register& rd, uint64_t imm, int shift = -1) {
     VIXL_ASSERT(!rd.IsZero());