[llvm] [AMDGPU] Fix s_singleuse_vdst not detecing exec mask changes (PR #89401)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-amdgpu

Author: Scott Egerton (ScottEgerton)

<details>
<summary>Changes</summary>

Instead of erasing unused registers, setting them
to 0 fixes an issue where single use values were
being incorrectly detected across changes to the
exec mask.
This issue was occuring because erasing the
values was causing the algorithm to forget which
values had been marked as invalid single use
candidates.


---
Full diff: https://github.com/llvm/llvm-project/pull/89401.diff


2 Files Affected:

- (modified) llvm/lib/Target/AMDGPU/AMDGPUInsertSingleUseVDST.cpp (+1-1) 
- (modified) llvm/test/CodeGen/AMDGPU/insert-singleuse-vdst.mir (-3) 


``````````diff
diff --git a/llvm/lib/Target/AMDGPU/AMDGPUInsertSingleUseVDST.cpp b/llvm/lib/Target/AMDGPU/AMDGPUInsertSingleUseVDST.cpp
index f6a63502ec2cda..1a7a01c1ea1821 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUInsertSingleUseVDST.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUInsertSingleUseVDST.cpp
@@ -112,7 +112,7 @@ class AMDGPUInsertSingleUseVDST : public MachineFunctionPass {
 
           // Reset uses count when a register is no longer live.
           for (const MCRegUnit &Unit : RegUnits)
-            RegisterUseCount.erase(Unit);
+            RegisterUseCount[Unit] = 0;
         }
         if (AllProducerOperandsAreSingleUse && SIInstrInfo::isVALU(MI)) {
           // TODO: Replace with candidate logging for instruction grouping
diff --git a/llvm/test/CodeGen/AMDGPU/insert-singleuse-vdst.mir b/llvm/test/CodeGen/AMDGPU/insert-singleuse-vdst.mir
index 135a101822bfa6..f12a8e95593fe2 100644
--- a/llvm/test/CodeGen/AMDGPU/insert-singleuse-vdst.mir
+++ b/llvm/test/CodeGen/AMDGPU/insert-singleuse-vdst.mir
@@ -398,7 +398,6 @@ body: |
   ; CHECK-NEXT:   successors: %bb.1(0x80000000)
   ; CHECK-NEXT:   liveins: $sgpr0_sgpr1
   ; CHECK-NEXT: {{  $}}
-  ; CHECK-NEXT:   S_SINGLEUSE_VDST 1
   ; CHECK-NEXT:   $vgpr0 = V_MOV_B32_e32 0, implicit $exec
   ; CHECK-NEXT:   $exec = COPY $sgpr0_sgpr1
   ; CHECK-NEXT:   $vgpr0 = V_MOV_B32_e32 $vgpr0, implicit $exec
@@ -424,7 +423,6 @@ body: |
   ; CHECK-NEXT:   successors: %bb.1(0x80000000)
   ; CHECK-NEXT:   liveins: $sgpr0
   ; CHECK-NEXT: {{  $}}
-  ; CHECK-NEXT:   S_SINGLEUSE_VDST 1
   ; CHECK-NEXT:   $vgpr0 = V_MOV_B32_e32 0, implicit $exec
   ; CHECK-NEXT:   $exec_lo = COPY $sgpr0
   ; CHECK-NEXT:   $vgpr0 = V_MOV_B32_e32 $vgpr0, implicit $exec
@@ -450,7 +448,6 @@ body: |
   ; CHECK-NEXT:   successors: %bb.1(0x80000000)
   ; CHECK-NEXT:   liveins: $sgpr0
   ; CHECK-NEXT: {{  $}}
-  ; CHECK-NEXT:   S_SINGLEUSE_VDST 1
   ; CHECK-NEXT:   $vgpr0 = V_MOV_B32_e32 0, implicit $exec
   ; CHECK-NEXT:   $exec_hi = COPY $sgpr0
   ; CHECK-NEXT:   $vgpr0 = V_MOV_B32_e32 $vgpr0, implicit $exec

``````````

</details>


https://github.com/llvm/llvm-project/pull/89401