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

[Scott Egerton via llvm-commits]

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

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.


>From 361549a77c88af24e6fee58d5c608b96befa69ed Mon Sep 17 00:00:00 2001
From: Scott Egerton <scott.egerton at amd.com>
Date: Fri, 19 Apr 2024 13:14:22 +0100
Subject: [PATCH] [AMDGPU] Fix s_singleuse_vdst not detecing exec mask changes

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.
---
 llvm/lib/Target/AMDGPU/AMDGPUInsertSingleUseVDST.cpp | 2 +-
 llvm/test/CodeGen/AMDGPU/insert-singleuse-vdst.mir   | 3 ---
 2 files changed, 1 insertion(+), 4 deletions(-)

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