[AArch64] Enable dead register definitions at O0 #145174
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@llvm/pr-subscribers-llvm-globalisel @llvm/pr-subscribers-backend-aarch64 Author: AZero13 (AZero13) ChangesThis is because the assembler/disassembler will otherwise show this as an adds or subs and not cmp or cmn. Even gcc does this. And, this is just allocating register to 0 reg. Will this optimize -O0? Well, minimally it will put less register pressure maybe, but that is not the point. The point is that it should resolve to aliases for better understanding, which is why -O0 exists to begin with. Full diff: https://github.com/llvm/llvm-project/pull/145174.diff 2 Files Affected:
diff --git a/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp b/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp
index 8150e91c8ba52..cec2c1b8374c6 100644
--- a/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp
+++ b/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp
@@ -803,8 +803,8 @@ bool AArch64PassConfig::addILPOpts() {
void AArch64PassConfig::addPreRegAlloc() {
// Change dead register definitions to refer to the zero register.
- if (TM->getOptLevel() != CodeGenOptLevel::None &&
- EnableDeadRegisterElimination)
+ // This is beneficial even at -O0 as we can show CMP/CMN in the assembler output.
+ if (EnableDeadRegisterElimination)
addPass(createAArch64DeadRegisterDefinitions());
// Use AdvSIMD scalar instructions whenever profitable.
diff --git a/llvm/test/CodeGen/AArch64/fast-isel-O0-cmp b/llvm/test/CodeGen/AArch64/fast-isel-O0-cmp
new file mode 100644
index 0000000000000..e5d97df40db25
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/fast-isel-O0-cmp
@@ -0,0 +1,38 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -mtriple=aarch64-linux-gnu -O0 -fast-isel -fast-isel-abort=1 -verify-machineinstrs < %s | FileCheck %s --check-prefix=CHECK
+
+; even in -O0, cmp should be cmp
+define i1 @cmp(i32 %0) {
+; CHECK-LABEL: cmp:
+; CHECK: // %bb.0:
+; CHECK-NEXT: cmp w0, #5
+; CHECK-NEXT: cset w0, gt
+; CHECK-NEXT: // kill: def $w1 killed $w0
+; CHECK-NEXT: ret
+ %2 = icmp sgt i32 %0, 5
+ ret i1 %2
+}
+
+define i1 @cmn(i32 %0) {
+; CHECK-LABEL: cmn:
+; CHECK: // %bb.0:
+; CHECK-NEXT: cmn w0, #5
+; CHECK-NEXT: cset w0, gt
+; CHECK-NEXT: // kill: def $w1 killed $w0
+; CHECK-NEXT: ret
+ %2 = icmp sgt i32 %0, -5
+ ret i1 %2
+}
+
+; Test that 0 is cmp
+define i1 @cmp0(i32 %0) {
+; CHECK-LABEL: cmp0:
+; CHECK: // %bb.0:
+; CHECK-NEXT: cmp w0, #0
+; CHECK-NEXT: cset w0, gt
+; CHECK-NEXT: // kill: def $w1 killed $w0
+; CHECK-NEXT: ret
+ %2 = icmp sgt i32 %0, 0
+ ret i1 %2
+}
+
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✅ With the latest revision this PR passed the C/C++ code formatter. |
This is because the assembler/disassembler will otherwise show this as an adds or subs and not cmp or cmn. Even gcc does this. And, this is just allocating register to 0 reg. Will this optimize -O0? Well, minimally it will put less register pressure maybe, but that is not the point. The point is that it should resolve to aliases for better understanding, which is why -O0 exists to begin with.
arsenm
reviewed
Jun 22, 2025
|
You know I've thought about it. This is a bad decision. I'll just work on another solution is fast-isel. Sorry about this. |
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This is because the assembler/disassembler will otherwise show this as an adds or subs and not cmp or cmn. Even gcc does this. And, this is just allocating register to 0 reg. Will this optimize -O0? Well, minimally it will put less register pressure maybe, but that is not the point. The point is that it should resolve to aliases for better understanding, which is why -O0 exists to begin with.