[llvm] [AMDGPU][MC] Fix a crash when invalid SDWA encoding is used (PR #215140)

[Shilei Tian via llvm-commits]

https://github.com/shiltian created https://github.com/llvm/llvm-project/pull/215140

Fixes #215006.


>From 113befc2cdc76abd9cb0ef7fa74918e92d18df2c Mon Sep 17 00:00:00 2001
From: Shilei Tian <i at tianshilei.me>
Date: Sun, 9 Aug 2026 16:10:28 -0400
Subject: [PATCH] [AMDGPU][MC] Fix a crash when invalid SDWA encoding is used

Fixes #215006.
---
 .../Disassembler/AMDGPUDisassembler.cpp       | 23 +++++++++++++++++++
 llvm/lib/Target/AMDGPU/SIInstrInfo.td         |  5 +++-
 .../MC/Disassembler/AMDGPU/decode-err.txt     | 14 +++++++++++
 3 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp b/llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp
index d05997cacba0a..9c70d01abe225 100644
--- a/llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp
+++ b/llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp
@@ -453,6 +453,29 @@ DECODE_SDWA(Src32)
 DECODE_SDWA(Src16)
 DECODE_SDWA(VopcDst)
 
+// The 3-bit SDWA sel fields only define values up to DWORD; 7 is reserved.
+static DecodeStatus decodeSDWASel(MCInst &Inst, unsigned Imm,
+                                  uint64_t /* Addr */,
+                                  const MCDisassembler * /* Decoder */) {
+  using namespace AMDGPU::SDWA;
+
+  if (Imm > SdwaSel::DWORD)
+    return MCDisassembler::Fail;
+  return addOperand(Inst, MCOperand::createImm(Imm));
+}
+
+// The 2-bit SDWA dst_unused field only defines values up to UNUSED_PRESERVE;
+// 3 is reserved.
+static DecodeStatus decodeSDWADstUnused(MCInst &Inst, unsigned Imm,
+                                        uint64_t /* Addr */,
+                                        const MCDisassembler * /* Decoder */) {
+  using namespace AMDGPU::SDWA;
+
+  if (Imm > DstUnused::UNUSED_PRESERVE)
+    return MCDisassembler::Fail;
+  return addOperand(Inst, MCOperand::createImm(Imm));
+}
+
 static DecodeStatus decodeVersionImm(MCInst &Inst, unsigned Imm,
                                      uint64_t /* Addr */,
                                      const MCDisassembler *Decoder) {
diff --git a/llvm/lib/Target/AMDGPU/SIInstrInfo.td b/llvm/lib/Target/AMDGPU/SIInstrInfo.td
index 5baf52dd12407..1f05bf8961f3b 100644
--- a/llvm/lib/Target/AMDGPU/SIInstrInfo.td
+++ b/llvm/lib/Target/AMDGPU/SIInstrInfo.td
@@ -1247,6 +1247,7 @@ class SDWAOperand<string Id, string Name = NAME>
   let ParserMethod =
     "[this](OperandVector &Operands) -> ParseStatus { "#
     "return parseSDWASel(Operands, \""#Id#"\", AMDGPUOperand::"#ImmTy#"); }";
+  let DecoderMethod = "decodeSDWASel";
 }
 
 class ArrayOperand0<string Id, string Name = NAME>
@@ -1312,7 +1313,9 @@ def Dim : CustomOperand</*optional=*/1, type=i8>;
 def dst_sel : SDWAOperand<"dst_sel", "SDWADstSel">;
 def src0_sel : SDWAOperand<"src0_sel", "SDWASrc0Sel">;
 def src1_sel : SDWAOperand<"src1_sel", "SDWASrc1Sel">;
-def dst_unused : CustomOperand<1, "SDWADstUnused">;
+def dst_unused : CustomOperand<1, "SDWADstUnused"> {
+  let DecoderMethod = "decodeSDWADstUnused";
+}
 
 def op_sel0 : ArrayOperand0<"op_sel", "OpSel">;
 def op_sel_hi0 : ArrayOperand0<"op_sel_hi", "OpSelHi">;
diff --git a/llvm/test/MC/Disassembler/AMDGPU/decode-err.txt b/llvm/test/MC/Disassembler/AMDGPU/decode-err.txt
index de9bf7bac9548..b4b65d029ce19 100644
--- a/llvm/test/MC/Disassembler/AMDGPU/decode-err.txt
+++ b/llvm/test/MC/Disassembler/AMDGPU/decode-err.txt
@@ -61,6 +61,20 @@
 # GFX1250-ERR: [[@LINE+1]]:1: warning: invalid instruction encoding
 0xfc,0x0e,0x80,0xbe
 
+# These are v_add_f32_sdwa with a reserved value in one of the SDWA fields:
+# 7 in a 3-bit sel field and 3 in the 2-bit dst_unused field.
+# GCN-ERR: [[@LINE+1]]:1: warning: invalid instruction encoding
+0xf9,0x04,0x00,0x02,0x01,0x07,0x06,0x06
+
+# GCN-ERR: [[@LINE+1]]:1: warning: invalid instruction encoding
+0xf9,0x04,0x00,0x02,0x01,0x1e,0x06,0x06
+
+# GCN-ERR: [[@LINE+1]]:1: warning: invalid instruction encoding
+0xf9,0x04,0x00,0x02,0x01,0x06,0x07,0x06
+
+# GCN-ERR: [[@LINE+1]]:1: warning: invalid instruction encoding
+0xf9,0x04,0x00,0x02,0x01,0x06,0x06,0x07
+
 # W32: v_dual_add_f32 v5, 0xaf123456, v2 :: v_dual_fmaak_f32 v6, v3, v1, 0xaf123456 ; encoding: [0xff,0x04,0x02,0xc9,0x03,0x03,0x06,0x05,0x56,0x34,0x12,0xaf]
 # W64: [[@LINE+1]]:1: warning: invalid instruction encoding
 0xff,0x04,0x02,0xc9,0x03,0x03,0x06,0x05,0x56,0x34,0x12,0xaf