From d64c0605ea07128a7bbcb648dab5ad8a242ffef5 Mon Sep 17 00:00:00 2001
From: Oguzhan Ozdemir <48760055+oguzhanozfe@users.noreply.github.com>
Date: Wed, 7 Oct 2026 18:51:56 +0300
Subject: [PATCH] fix(relinker): let REX.W override 0x66 when sizing decoder
 immediates

In 64-bit mode REX.W takes precedence over the 0x66 operand-size prefix,
so the immediate of 81, C7, 69, 68, A9, the 05..3D accumulator forms and
the F7 TEST form stays imm32. The decoder only applied this to B8+r and
returned a length two bytes short for the others.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 .../relinker/codegen/src/x86/X64InstructionDecoder.cpp | 10 ++++++----
 .../relinker/codegen/tests/Amd64OnlyConverterTests.cpp |  4 ++++
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/core/relinker/codegen/src/x86/X64InstructionDecoder.cpp b/core/relinker/codegen/src/x86/X64InstructionDecoder.cpp
index 065c7f9dd..9cc934523 100644
--- a/core/relinker/codegen/src/x86/X64InstructionDecoder.cpp
+++ b/core/relinker/codegen/src/x86/X64InstructionDecoder.cpp
@@ -39,6 +39,9 @@ std::size_t X64InstructionDecoder::Decode(const std::uint8_t* data, std::size_t
         pos += 1;
     }
 
+    const bool rexW = rexPresent && (rex & RexWBit) != 0;
+    const std::size_t operandImmediateSize = operandSizeOverride && !rexW ? ImmSize16 : ImmSize32;
+
     if (pos >= available) {
         throw CodegenException("Instruction truncated after prefixes");
     }
@@ -172,10 +175,9 @@ std::size_t X64InstructionDecoder::Decode(const std::uint8_t* data, std::size_t
                    opcode == OneByteTestEaxImm32 ||
                    (opcode >= OneByteMovImm32RegMin && opcode <= OneByteMovImm32RegMax)) {
             if (opcode >= OneByteMovImm32RegMin && opcode <= OneByteMovImm32RegMax) {
-                immediateSize = (rexPresent && (rex & RexWBit) != 0) ? ImmSize64 :
-                    (operandSizeOverride ? ImmSize16 : ImmSize32);
+                immediateSize = rexW ? ImmSize64 : operandImmediateSize;
             } else {
-                immediateSize = operandSizeOverride ? ImmSize16 : ImmSize32;
+                immediateSize = operandImmediateSize;
             }
         } else if (opcode == OneByteImm8Grp1 || opcode == OneByteImulRm32Imm8) {
             immediateSize = ImmSize8;
@@ -279,7 +281,7 @@ std::size_t X64InstructionDecoder::Decode(const std::uint8_t* data, std::size_t
         reg <= Grp3RegTestMax) {
         immediateSize = (opcode == OneByteTestGrp3Rm8)
             ? ImmSize8
-            : (operandSizeOverride ? ImmSize16 : ImmSize32);
+            : operandImmediateSize;
     }
 
     if (mod != ModRmModRegister && rm == ModRmRmSibPresent) {
diff --git a/core/relinker/codegen/tests/Amd64OnlyConverterTests.cpp b/core/relinker/codegen/tests/Amd64OnlyConverterTests.cpp
index c4fab89f4..65e297c29 100644
--- a/core/relinker/codegen/tests/Amd64OnlyConverterTests.cpp
+++ b/core/relinker/codegen/tests/Amd64OnlyConverterTests.cpp
@@ -127,6 +127,10 @@ void decoderLengths() {
         {0x0F, 0x38, 0xCB, 0xCA}, {0x45, 0x0F, 0x38, 0xCC, 0xE1}, {0x0F, 0x38, 0xCD, 0x08}, {0x0F, 0x38, 0xCB, 0x0D, 0x10, 0x00, 0x00, 0x00},
         {0x0F, 0x3A, 0xCC, 0xD5, 0x03}, {0x45, 0x0F, 0x3A, 0xCC, 0xE1, 0x00}, {0x0F, 0x3A, 0xCC, 0x08, 0x01}, {0x0F, 0x38, 0xC8, 0xCA}, {0x0F, 0x38, 0xC9, 0xCA}, {0x0F, 0x38, 0xCA, 0x08},
         {0x48, 0x66, 0xB8, 0x34, 0x12}, {0x66, 0x48, 0xB8, 1, 2, 3, 4, 5, 6, 7, 8}, {0x41, 0x48, 0xB8, 1, 2, 3, 4, 5, 6, 7, 8},
+        {0x66, 0x48, 0x81, 0xC0, 1, 2, 3, 4}, {0x66, 0x48, 0x05, 1, 2, 3, 4}, {0x66, 0x48, 0xC7, 0xC0, 1, 2, 3, 4},
+        {0x66, 0x48, 0x69, 0xC0, 1, 2, 3, 4}, {0x66, 0x48, 0x68, 1, 2, 3, 4}, {0x66, 0x48, 0xA9, 1, 2, 3, 4},
+        {0x66, 0x48, 0xF7, 0xC0, 1, 2, 3, 4}, {0x66, 0x81, 0xC0, 0x34, 0x12}, {0x48, 0x66, 0x81, 0xC0, 0x34, 0x12},
+        {0x66, 0x41, 0x81, 0xC0, 0x34, 0x12}, {0x66, 0xF7, 0xC0, 0x34, 0x12},
         {0x48, 0x64, 0x8B, 0x00}, {0x48, 0xF3, 0x0F, 0x2B, 0x00}, {0x48, 0x67, 0x0F, 0x01, 0xFC}, {0x48, 0x48, 0x0F, 0x01, 0xFC}};
     Bytes padded;
     for (const auto& instruction : instructions) {