[llvm] AMDGPU/MC: Try harder to evaluate absolute MC expressions (PR #145146)

[Nicolai Hähnle via llvm-commits]

https://github.com/nhaehnle created https://github.com/llvm/llvm-project/pull/145146

This is a follow-up to commit 24c860547e8 ("AMDGPU/MC: Fix emitting absolute expressions (#136789)").

In some downstream work, we end up with an MCTargetExpr that is a maximum (AGVK_Max) in an instruction operand. getMachineOpValueCommon recognizes the absolute nature of the expression and doesn't emit a fixup. getLitEncoding needs to be aligned with this decision, else we end up with a 0 immediate without a corresponding fixup.

Note that evaluateAsAbsolute checks for MCConstantExpr as a fast path, so this accepts strictly more cases than before.

I've tried several ways to write a test for this without success. The challenge is that there is no upstream way to generate this kind of expression in an instruction operand natively, and trying to create one via inline assembly fails because the assembly parser evaluates the expression to a constant during parsing.

>From 1032b0642fc176e59e3cfdcf7439fe5178060d19 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nicolai=20H=C3=A4hnle?= <nicolai.haehnle at amd.com>
Date: Fri, 20 Jun 2025 20:03:06 -0700
Subject: [PATCH] AMDGPU/MC: Try harder to evaluate absolute MC expressions

This is a follow-up to commit 24c860547e8 ("AMDGPU/MC: Fix emitting
absolute expressions (#136789)").

In some downstream work, we end up with an MCTargetExpr that is a
maximum (AGVK_Max) in an instruction operand. getMachineOpValueCommon
recognizes the absolute nature of the expression and doesn't emit a
fixup. getLitEncoding needs to be aligned with this decision, else we
end up with a 0 immediate without a corresponding fixup.

Note that evaluateAsAbsolute checks for MCConstantExpr as a fast path,
so this accepts strictly more cases than before.

I've tried several ways to write a test for this without success. The
challenge is that there is no upstream way to generate this kind of
expression in an instruction operand natively, and trying to create one
via inline assembly fails because the assembly parser evaluates the
expression to a constant during parsing.
---
 llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUMCCodeEmitter.cpp | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUMCCodeEmitter.cpp b/llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUMCCodeEmitter.cpp
index ab7e51ac28322..abb5003f3320b 100644
--- a/llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUMCCodeEmitter.cpp
+++ b/llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUMCCodeEmitter.cpp
@@ -255,13 +255,9 @@ AMDGPUMCCodeEmitter::getLitEncoding(const MCOperand &MO,
                                     const MCSubtargetInfo &STI) const {
   int64_t Imm;
   if (MO.isExpr()) {
-    const auto *C = dyn_cast<MCConstantExpr>(MO.getExpr());
-    if (!C)
+    if (!MO.getExpr()->evaluateAsAbsolute(Imm))
       return 255;
-
-    Imm = C->getValue();
   } else {
-
     assert(!MO.isDFPImm());
 
     if (!MO.isImm())