[llvm] [RISCV] Move RISCVDeadRegisterDefinitions to post regalloc (PR #90636)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-risc-v

Author: Luke Lau (lukel97)

<details>
<summary>Changes</summary>

The original patch mentions that it may reduce register pressure and improve register allocation, but I believe the register allocator already accounts for dead defs hence why this almost an NFC.

There are no differences on llvm-test-suite or SPEC CPU 2017 with -O3 and -march=rv64gv, just one case in the in-tree tests where a vsetvli output VL is now marked as dead after phi elimination.

The motivation for this is to slightly simplify how we handle vleff with post (vector) regalloc in #<!-- -->70549, by keeping GPR defs virtual where possible.


---
Full diff: https://github.com/llvm/llvm-project/pull/90636.diff


4 Files Affected:

- (modified) llvm/lib/Target/RISCV/RISCVDeadRegisterDefinitions.cpp (+1-3) 
- (modified) llvm/lib/Target/RISCV/RISCVTargetMachine.cpp (+3-3) 
- (modified) llvm/test/CodeGen/RISCV/O3-pipeline.ll (+1-1) 
- (modified) llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.ll (+2-2) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/RISCVDeadRegisterDefinitions.cpp b/llvm/lib/Target/RISCV/RISCVDeadRegisterDefinitions.cpp
index df607236f7d56f..fd6708c44e7720 100644
--- a/llvm/lib/Target/RISCV/RISCVDeadRegisterDefinitions.cpp
+++ b/llvm/lib/Target/RISCV/RISCVDeadRegisterDefinitions.cpp
@@ -77,10 +77,8 @@ bool RISCVDeadRegisterDefinitions::runOnMachineFunction(MachineFunction &MF) {
           LLVM_DEBUG(dbgs() << "    Ignoring, def is tied operand.\n");
           continue;
         }
-        // We should not have any relevant physreg defs that are replacable by
-        // zero before register allocation. So we just check for dead vreg defs.
         Register Reg = MO.getReg();
-        if (!Reg.isVirtual() || (!MO.isDead() && !MRI->use_nodbg_empty(Reg)))
+        if (!MO.isDead() && !MRI->use_nodbg_empty(Reg))
           continue;
         LLVM_DEBUG(dbgs() << "    Dead def operand #" << I << " in:\n      ";
                    MI.print(dbgs()));
diff --git a/llvm/lib/Target/RISCV/RISCVTargetMachine.cpp b/llvm/lib/Target/RISCV/RISCVTargetMachine.cpp
index 0876f46728a10c..d4978abf205f53 100644
--- a/llvm/lib/Target/RISCV/RISCVTargetMachine.cpp
+++ b/llvm/lib/Target/RISCV/RISCVTargetMachine.cpp
@@ -536,9 +536,6 @@ void RISCVPassConfig::addPreRegAlloc() {
   if (TM->getOptLevel() != CodeGenOptLevel::None)
     addPass(createRISCVMergeBaseOffsetOptPass());
   addPass(createRISCVInsertVSETVLIPass());
-  if (TM->getOptLevel() != CodeGenOptLevel::None &&
-      EnableRISCVDeadRegisterElimination)
-    addPass(createRISCVDeadRegisterDefinitionsPass());
   addPass(createRISCVInsertReadWriteCSRPass());
   addPass(createRISCVInsertWriteVXRMPass());
 }
@@ -553,6 +550,9 @@ void RISCVPassConfig::addPostRegAlloc() {
   if (TM->getOptLevel() != CodeGenOptLevel::None &&
       EnableRedundantCopyElimination)
     addPass(createRISCVRedundantCopyEliminationPass());
+  if (TM->getOptLevel() != CodeGenOptLevel::None &&
+      EnableRISCVDeadRegisterElimination)
+    addPass(createRISCVDeadRegisterDefinitionsPass());
 }
 
 yaml::MachineFunctionInfo *
diff --git a/llvm/test/CodeGen/RISCV/O3-pipeline.ll b/llvm/test/CodeGen/RISCV/O3-pipeline.ll
index 4121d111091117..174996b9c10b00 100644
--- a/llvm/test/CodeGen/RISCV/O3-pipeline.ll
+++ b/llvm/test/CodeGen/RISCV/O3-pipeline.ll
@@ -116,7 +116,6 @@
 ; CHECK-NEXT:       RISC-V Pre-RA pseudo instruction expansion pass
 ; CHECK-NEXT:       RISC-V Merge Base Offset
 ; CHECK-NEXT:       RISC-V Insert VSETVLI pass
-; CHECK-NEXT:       RISC-V Dead register definitions
 ; CHECK-NEXT:       RISC-V Insert Read/Write CSR Pass
 ; CHECK-NEXT:       RISC-V Insert Write VXRM Pass
 ; CHECK-NEXT:       Detect Dead Lanes
@@ -153,6 +152,7 @@
 ; CHECK-NEXT:       Machine Copy Propagation Pass
 ; CHECK-NEXT:       Machine Loop Invariant Code Motion
 ; CHECK-NEXT:       RISC-V Redundant Copy Elimination
+; CHECK-NEXT:       RISC-V Dead register definitions
 ; CHECK-NEXT:       Remove Redundant DEBUG_VALUE analysis
 ; CHECK-NEXT:       Fixup Statepoint Caller Saved
 ; CHECK-NEXT:       PostRA Machine Sink
diff --git a/llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.ll b/llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.ll
index 4ff2fc7a5fff5d..088d121564bc96 100644
--- a/llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.ll
+++ b/llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.ll
@@ -88,13 +88,13 @@ define <vscale x 1 x double> @test3(i64 %avl, i8 zeroext %cond, <vscale x 1 x do
 ; CHECK:       # %bb.0: # %entry
 ; CHECK-NEXT:    beqz a1, .LBB2_2
 ; CHECK-NEXT:  # %bb.1: # %if.then
-; CHECK-NEXT:    vsetvli a0, a0, e64, m1, ta, ma
+; CHECK-NEXT:    vsetvli zero, a0, e64, m1, ta, ma
 ; CHECK-NEXT:    vfadd.vv v9, v8, v9
 ; CHECK-NEXT:    vfmul.vv v8, v9, v8
 ; CHECK-NEXT:    # implicit-def: $x10
 ; CHECK-NEXT:    ret
 ; CHECK-NEXT:  .LBB2_2: # %if.else
-; CHECK-NEXT:    vsetvli a0, a0, e64, m1, ta, ma
+; CHECK-NEXT:    vsetvli zero, a0, e64, m1, ta, ma
 ; CHECK-NEXT:    vfsub.vv v9, v8, v9
 ; CHECK-NEXT:    vfmul.vv v8, v9, v8
 ; CHECK-NEXT:    # implicit-def: $x10

``````````

</details>


https://github.com/llvm/llvm-project/pull/90636