[llvm] dabb0dd - [MCP] Skip invalidating def constant regs during forward propagation (#111129)
via llvm-commits
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Thu Oct 10 07:05:47 PDT 2024
Author: Vladimir Radosavljevic
Date: 2024-10-10T18:05:42+04:00
New Revision: dabb0ddbd7a7229855156c61df1d35ad845361ac
URL: https://github.com/llvm/llvm-project/commit/dabb0ddbd7a7229855156c61df1d35ad845361ac
DIFF: https://github.com/llvm/llvm-project/commit/dabb0ddbd7a7229855156c61df1d35ad845361ac.diff
LOG: [MCP] Skip invalidating def constant regs during forward propagation (#111129)
Before this patch, redundant COPY couldn't be removed for the following
case:
```
%reg1 = COPY %const-reg
... // There is a def of %const-reg
%reg2 = COPY killed %reg1
```
where this can be optimized to:
```
... // There is a def of %const-reg
%reg2 = COPY %const-reg
```
This patch allows for such optimization by not invalidating defined
constant registers. This is safe, as architectures like AArch64 and
RISCV replace a dead definition of a GPR with a zero constant register
for certain instructions.
Added:
llvm/test/CodeGen/AArch64/machine-cp-constant-reg.mir
Modified:
llvm/lib/CodeGen/MachineCopyPropagation.cpp
Removed:
################################################################################
diff --git a/llvm/lib/CodeGen/MachineCopyPropagation.cpp b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
index 8bcc437cbfb865..fb4da2c11cda77 100644
--- a/llvm/lib/CodeGen/MachineCopyPropagation.cpp
+++ b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
@@ -886,8 +886,11 @@ void MachineCopyPropagation::ForwardCopyPropagateBlock(MachineBasicBlock &MBB) {
"MachineCopyPropagation should be run after register allocation!");
if (MO.isDef() && !MO.isEarlyClobber()) {
- Defs.push_back(Reg.asMCReg());
- continue;
+ // Skip invalidating constant registers.
+ if (!MRI->isConstantPhysReg(Reg)) {
+ Defs.push_back(Reg.asMCReg());
+ continue;
+ }
} else if (MO.readsReg())
ReadRegister(Reg.asMCReg(), MI, MO.isDebug() ? DebugUse : RegularUse);
}
diff --git a/llvm/test/CodeGen/AArch64/machine-cp-constant-reg.mir b/llvm/test/CodeGen/AArch64/machine-cp-constant-reg.mir
new file mode 100644
index 00000000000000..cad55b9daffafd
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/machine-cp-constant-reg.mir
@@ -0,0 +1,19 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
+# RUN: llc -mtriple=aarch64-none-linux-gnu -run-pass machine-cp -verify-machineinstrs -o - %s | FileCheck %s
+
+---
+name: test
+body: |
+ bb.0:
+ liveins: $w2
+ ; CHECK-LABEL: name: test
+ ; CHECK: liveins: $w2
+ ; CHECK-NEXT: {{ $}}
+ ; CHECK-NEXT: $wzr = SUBSWri killed renamable $w2, 0, 0, implicit-def $nzcv
+ ; CHECK-NEXT: renamable $w0 = COPY $wzr
+ ; CHECK-NEXT: RET_ReallyLR implicit killed $w0
+ renamable $w1 = COPY $wzr
+ $wzr = SUBSWri killed renamable $w2, 0, 0, implicit-def $nzcv
+ renamable $w0 = COPY killed renamable $w1
+ RET_ReallyLR implicit killed $w0
+...
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