[llvm] Mips: Mark MUL's existing HI0/LO0 defs dead in FastISel (PR #227230)

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llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-mips

Author: Matt Arsenault (arsenm)

<details>
<summary>Changes</summary>

Instead of appending a second, dead pair of implicit defs on top of the
ones the instruction description already provides, set the dead flag on
the existing operands. This drops the duplicate operands and avoids
later re-inference of dead.

Co-Authored-By: Claude Opus 5 <noreply@<!-- -->anthropic.com>

---
Full diff: https://github.com/llvm/llvm-project/pull/227230.diff


2 Files Affected:

- (modified) llvm/lib/Target/Mips/MipsFastISel.cpp (+4-4) 
- (added) llvm/test/CodeGen/Mips/Fast-ISel/mul-dead-hilo.ll (+19) 


``````````diff
diff --git a/llvm/lib/Target/Mips/MipsFastISel.cpp b/llvm/lib/Target/Mips/MipsFastISel.cpp
index 8e79160c131f3..7d1f024002eaf 100644
--- a/llvm/lib/Target/Mips/MipsFastISel.cpp
+++ b/llvm/lib/Target/Mips/MipsFastISel.cpp
@@ -2147,10 +2147,10 @@ unsigned MipsFastISel::fastEmitInst_rr(unsigned MachineInstOpcode,
     Op0 = constrainOperandRegClass(II, Op0, II.getNumDefs());
     Op1 = constrainOperandRegClass(II, Op1, II.getNumDefs() + 1);
     BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, MIMD, II, ResultReg)
-      .addReg(Op0)
-      .addReg(Op1)
-      .addReg(Mips::HI0, RegState::ImplicitDefine | RegState::Dead)
-      .addReg(Mips::LO0, RegState::ImplicitDefine | RegState::Dead);
+        .addReg(Op0)
+        .addReg(Op1)
+        .setOperandDead(3)  // implicit-def $hi0
+        .setOperandDead(4); // implicit-def $lo0
     return ResultReg;
   }
 
diff --git a/llvm/test/CodeGen/Mips/Fast-ISel/mul-dead-hilo.ll b/llvm/test/CodeGen/Mips/Fast-ISel/mul-dead-hilo.ll
new file mode 100644
index 0000000000000..2f7e661f3d58f
--- /dev/null
+++ b/llvm/test/CodeGen/Mips/Fast-ISel/mul-dead-hilo.ll
@@ -0,0 +1,19 @@
+; NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 6
+; RUN: llc -mtriple=mipsel -mcpu=mips32 -O0 -relocation-model=pic -stop-after=finalize-isel -o - < %s | FileCheck %s
+
+define i32 @mul(i32 %a, i32 %b) {
+  ; CHECK-LABEL: name: mul
+  ; CHECK: bb.0.entry:
+  ; CHECK-NEXT:   liveins: $a0, $a1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:gpr32 = COPY $a1
+  ; CHECK-NEXT:   [[COPY1:%[0-9]+]]:gpr32 = COPY $a0
+  ; CHECK-NEXT:   [[COPY2:%[0-9]+]]:gpr32 = COPY killed [[COPY1]]
+  ; CHECK-NEXT:   [[COPY3:%[0-9]+]]:gpr32 = COPY killed [[COPY]]
+  ; CHECK-NEXT:   [[MUL:%[0-9]+]]:gpr32 = MUL [[COPY2]], [[COPY3]], implicit-def dead $hi0, implicit-def dead $lo0
+  ; CHECK-NEXT:   $v0 = COPY [[MUL]]
+  ; CHECK-NEXT:   RetRA implicit $v0
+entry:
+  %0 = mul i32 %a, %b
+  ret i32 %0
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/227230