AMDGPU/MC: Fix emitting absolute expressions

[nhaehnle]

When absolute MCExprs appear in normal instruction operands, we have to emit them like a normal inline constant or literal. More generally, an MCExpr that happens to have an absolute evaluation should be treated exactly like an immediate operand here.

No test; I found this downstream, and I don't think it can be triggered upstream yet.

Fixes: 1623866 ("[AMDGPU][MC] Support UC_VERSION_* constants. (#95618)")

[llvmbot]

@llvm/pr-subscribers-backend-amdgpu

Author: Nicolai Hähnle (nhaehnle)

Changes

When absolute MCExprs appear in normal instruction operands, we have to emit them like a normal inline constant or literal. More generally, an MCExpr that happens to have an absolute evaluation should be treated exactly like an immediate operand here.

No test; I found this downstream, and I don't think it can be triggered upstream yet.

Fixes: 1623866 ("[AMDGPU][MC] Support UC_VERSION_* constants. (#95618)")

---

Full diff: https://github.com/llvm/llvm-project/pull/136789.diff

1 Files Affected:

• (modified) llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUMCCodeEmitter.cpp (+13-7)

diff --git a/llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUMCCodeEmitter.cpp b/llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUMCCodeEmitter.cpp
index 1e82ee36dc0eb..9cf712318bfa1 100644
--- a/llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUMCCodeEmitter.cpp
+++ b/llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUMCCodeEmitter.cpp
@@ -647,13 +647,15 @@ void AMDGPUMCCodeEmitter::getMachineOpValueT16Lo128(
 void AMDGPUMCCodeEmitter::getMachineOpValueCommon(
     const MCInst &MI, const MCOperand &MO, unsigned OpNo, APInt &Op,
     SmallVectorImpl<MCFixup> &Fixups, const MCSubtargetInfo &STI) const {
+  bool isLikeImm = false;
   int64_t Val;
-  if (MO.isExpr() && MO.getExpr()->evaluateAsAbsolute(Val)) {
-    Op = Val;
-    return;
-  }
 
-  if (MO.isExpr() && MO.getExpr()->getKind() != MCExpr::Constant) {
+  if (MO.isImm()) {
+    Val = MO.getImm();
+    isLikeImm = true;
+  } else if (MO.isExpr() && MO.getExpr()->evaluateAsAbsolute(Val)) {
+    isLikeImm = true;
+  } else if (MO.isExpr()) {
     // FIXME: If this is expression is PCRel or not should not depend on what
     // the expression looks like. Given that this is just a general expression,
     // it should probably be FK_Data_4 and whatever is producing
@@ -683,8 +685,12 @@ void AMDGPUMCCodeEmitter::getMachineOpValueCommon(
       Op = *Enc;
       return;
     }
-  } else if (MO.isImm()) {
-    Op = MO.getImm();
+
+    llvm_unreachable("Operand not supported for SISrc");
+  }
+
+  if (isLikeImm) {
+    Op = Val;
     return;
   }
 

[arsenm]

Don't see why this isn't testable

[nhaehnle]

Don't see why this isn't testable

Any suggestions as to how? In the downstream case, this is triggered by a new MCResourceInfo::ResourceInfoKind which can be placed into an instruction operand. It's possible I missed something, but in main these enums only seem to be used in connection to the kernel descriptor. (And the code I'm changing was initially added for the MC-only purpose of supporting some s_version syntax.)

[arsenm]

Any suggestions as to how? In the downstream case, this is triggered by a new MCResourceInfo::ResourceInfoKind which can be placed into an instruction operand. It's possible I missed something, but in main these enums only seem to be used in connection to the kernel descriptor.

So emit a kernel descriptor?

[nhaehnle]

Any suggestions as to how? In the downstream case, this is triggered by a new MCResourceInfo::ResourceInfoKind which can be placed into an instruction operand. It's possible I missed something, but in main these enums only seem to be used in connection to the kernel descriptor.

So emit a kernel descriptor?

Maybe I misunderstand what you mean, but kernel descriptors don't contain instructions, do they?