From fa32940d1474631ed156c1a6fae4a3ffdd413317 Mon Sep 17 00:00:00 2001
From: nmzik <Nmzik@mail.ru>
Date: Wed, 21 Oct 2026 22:59:56 +0200
Subject: [PATCH] shader: drop BFM mask bits shifted past bit 31 instead of an
 out-of-range bit-field insert

---
 .../recompiler/frontend/translate/Integer.cpp |  5 ++--
 tests/ShaderRecompilerComputeTests.cpp        | 26 +++++++++++++++++++
 2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/src/graphics/shader/recompiler/frontend/translate/Integer.cpp b/src/graphics/shader/recompiler/frontend/translate/Integer.cpp
index 7fe8967c6..0a69f9e4c 100644
--- a/src/graphics/shader/recompiler/frontend/translate/Integer.cpp
+++ b/src/graphics/shader/recompiler/frontend/translate/Integer.cpp
@@ -380,9 +380,8 @@ void Translator::S_QUADMASK_B64(const Decoder::Instruction& inst) {
 void Translator::BFM_B32(const Decoder::Instruction& inst) {
 	const auto count  = ir.BitwiseAnd(ReadU32(inst.src0), IR::U32(IR::Value(31u)));
 	const auto offset = ir.BitwiseAnd(ReadU32(inst.src1), IR::U32(IR::Value(31u)));
-	const auto result = ir.Emit(IR::ValueOpcode::BitFieldInsert,
-	                            {IR::Value(0u), IR::Value(0xffffffffu), offset, count});
-	WriteOperand(DestinationOperand(inst), result);
+	// Mask bits shifted past bit 31 are dropped.
+	WriteOperand(DestinationOperand(inst), ir.ShiftLeftLogical(RightMask32(count), offset));
 }
 
 IR::U32 Translator::RightMask32(IR::U32 count) {
diff --git a/tests/ShaderRecompilerComputeTests.cpp b/tests/ShaderRecompilerComputeTests.cpp
index 97caf606a..0d4272408 100644
--- a/tests/ShaderRecompilerComputeTests.cpp
+++ b/tests/ShaderRecompilerComputeTests.cpp
@@ -25622,6 +25622,31 @@ TestCase VectorBfeI32SignExtendsField() {
   return test;
 }
 
+TestCase VectorBfmDropsMaskBitsPastBit31() {
+  using O = ShaderOpcode;
+
+  std::vector<u32> code;
+  for (u32 vgpr = 0; vgpr < 4; vgpr++) {
+    AppendVMovU32(&code, 30, vgpr * sizeof(u32));
+    AppendBufferLoadDword(&code, vgpr, 30);
+  }
+  AppendVop3(&code, 0x363, 10, Vgpr(0), Vgpr(1));
+  AppendVop3(&code, 0x363, 11, Vgpr(2), Vgpr(3));
+  AppendStoreVgpr(&code, 10, 0);
+  AppendStoreVgpr(&code, 11, 1);
+  AppendEnd(&code);
+
+  TestCase test;
+  test.name = "VectorBfmDropsMaskBitsPastBit31";
+  test.code = std::move(code);
+  // Size and offset use bits [4:0]; mask bits shifted past bit 31 are dropped.
+  test.initial = {24, 16, 0x3fu, 0x21u};
+  test.expected = {0xffff0000u, 0xfffffffeu};
+  test.opcodes = {O::V_MOV_B32, O::BUFFER_LOAD_DWORD, O::V_BFM_B32,
+                  O::BUFFER_STORE_DWORD, O::S_ENDPGM};
+  return test;
+}
+
 TestCase VectorAlignByteUsesTwoBitByteOffset() {
   using O = ShaderOpcode;
 
@@ -42981,6 +43006,7 @@ std::vector<TestCase> MakeCases() {
   AddCase(VectorVop3LshlrevB64Captured);
   AddCase(VectorVop3IntegerOps);
   AddCase(VectorBfeI32SignExtendsField);
+  AddCase(VectorBfmDropsMaskBitsPastBit31);
   AddCase(VectorAlignByteUsesTwoBitByteOffset);
   AddCase(VectorCarryAndBitCountOps);
   AddCase(ScalarVector64BitFloatInlineConstants);