[llvm] df96e09 - [StructurizeCFG] nested-if zerocost hoist bugfix (#155408)

[via llvm-commits]

Author: Vigneshwar Jayakumar
Date: 2025-08-28T09:04:14-05:00
New Revision: df96e09c1eec06555d54532a63c2317c4caddcf0

URL: https://github.com/llvm/llvm-project/commit/df96e09c1eec06555d54532a63c2317c4caddcf0
DIFF: https://github.com/llvm/llvm-project/commit/df96e09c1eec06555d54532a63c2317c4caddcf0.diff

LOG: [StructurizeCFG] nested-if zerocost hoist bugfix (#155408)

When zero cost instructions are hoisted, the simplifyHoistedPhi function
was setting incoming phi values which were not dominating the use
causing runtime failure. This was set to poison by rebuildSSA function.
This commit fixes the issue.

Added: 
    

Modified: 
    llvm/lib/Transforms/Scalar/StructurizeCFG.cpp
    llvm/test/CodeGen/AMDGPU/structurize-hoist.ll
    llvm/test/Transforms/StructurizeCFG/hoist-zerocost.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp b/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp
index bb7dbc2980f59..e05625344ee29 100644
--- a/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp
+++ b/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp
@@ -997,7 +997,8 @@ void StructurizeCFG::simplifyHoistedPhis() {
         continue;
 
       OtherPhi->setIncomingValue(PoisonValBBIdx, V);
-      Phi->setIncomingValue(i, OtherV);
+      if (DT->dominates(OtherV, Phi))
+        Phi->setIncomingValue(i, OtherV);
     }
   }
 }

diff  --git a/llvm/test/CodeGen/AMDGPU/structurize-hoist.ll b/llvm/test/CodeGen/AMDGPU/structurize-hoist.ll
index 8b6e5eb8e6975..b4036517cc0d5 100644
--- a/llvm/test/CodeGen/AMDGPU/structurize-hoist.ll
+++ b/llvm/test/CodeGen/AMDGPU/structurize-hoist.ll
@@ -189,12 +189,11 @@ define void @test_nested_if(ptr %ptr, i32 %val, i1 %cond) {
 ; GFX900-NEXT:    s_mov_b64 s[10:11], -1
 ; GFX900-NEXT:    s_xor_b64 s[4:5], s[6:7], -1
 ; GFX900-NEXT:    s_mov_b64 s[12:13], s[6:7]
-; GFX900-NEXT:    ; implicit-def: $vgpr3
+; GFX900-NEXT:    s_waitcnt vmcnt(0) lgkmcnt(0)
+; GFX900-NEXT:    v_mov_b32_e32 v3, v4
 ; GFX900-NEXT:    s_and_saveexec_b64 s[8:9], s[4:5]
 ; GFX900-NEXT:    s_cbranch_execz .LBB3_4
 ; GFX900-NEXT:  ; %bb.1: ; %if
-; GFX900-NEXT:    s_waitcnt vmcnt(0) lgkmcnt(0)
-; GFX900-NEXT:    v_mov_b32_e32 v3, v4
 ; GFX900-NEXT:    s_and_saveexec_b64 s[12:13], s[4:5]
 ; GFX900-NEXT:    s_cbranch_execz .LBB3_3
 ; GFX900-NEXT:  ; %bb.2: ; %if_2
@@ -230,7 +229,6 @@ define void @test_nested_if(ptr %ptr, i32 %val, i1 %cond) {
 ; GFX900-NEXT:    s_or_b64 exec, exec, s[8:9]
 ; GFX900-NEXT:    s_and_saveexec_b64 s[6:7], s[4:5]
 ; GFX900-NEXT:    s_or_b64 exec, exec, s[6:7]
-; GFX900-NEXT:    s_waitcnt vmcnt(0) lgkmcnt(0)
 ; GFX900-NEXT:    flat_store_dword v[0:1], v4
 ; GFX900-NEXT:    s_waitcnt vmcnt(0) lgkmcnt(0)
 ; GFX900-NEXT:    s_setpc_b64 s[30:31]

diff  --git a/llvm/test/Transforms/StructurizeCFG/hoist-zerocost.ll b/llvm/test/Transforms/StructurizeCFG/hoist-zerocost.ll
index 71de4f45f154f..d084e199ceb89 100644
--- a/llvm/test/Transforms/StructurizeCFG/hoist-zerocost.ll
+++ b/llvm/test/Transforms/StructurizeCFG/hoist-zerocost.ll
@@ -172,7 +172,7 @@ define void @test_nested_if(ptr %ptr, i32 %val, i1 %cond) {
 ; CHECK-NEXT:    [[A16:%.*]] = icmp slt i32 [[VAL]], 255
 ; CHECK-NEXT:    br i1 [[COND_INV]], label %[[THEN_2:.*]], label %[[FLOW1:.*]]
 ; CHECK:       [[FLOW]]:
-; CHECK-NEXT:    [[TMP0:%.*]] = phi i32 [ [[TMP2:%.*]], %[[FLOW1]] ], [ poison, %[[ENTRY]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = phi i32 [ [[TMP2:%.*]], %[[FLOW1]] ], [ [[A_ELSE]], %[[ENTRY]] ]
 ; CHECK-NEXT:    [[TMP1:%.*]] = phi i1 [ [[TMP3:%.*]], %[[FLOW1]] ], [ [[COND]], %[[ENTRY]] ]
 ; CHECK-NEXT:    br i1 [[TMP1]], label %[[ELSE:.*]], label %[[MERGE:.*]]
 ; CHECK:       [[THEN_2]]: