[llvm] [SimplifyCFG] Remove limitation on sinking of load/store of alloca (PR #104788)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Nikita Popov (nikic)

<details>
<summary>Changes</summary>

This is a followup to https://github.com/llvm/llvm-project/pull/104579 to remove the limitation on sinking loads/stores of allocas entirely, even if this would introduce a phi node.

Nowadays, SROA supports speculating load/store over select/phi. Additionally, SimplifyCFG with sinking only runs at the end of the function simplification pipeline, after SROA. I checked that the two tests modified here still successfully SROA after the SimplifyCFG transform.

We should, however, keep the limitation on lifetime intrinsics. SROA does not have speculation support for these, and I've also found that the way these are handled in the backend is very problematic (https://github.com/llvm/llvm-project/issues/104776), so I think we should leave them alone.

---
Full diff: https://github.com/llvm/llvm-project/pull/104788.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Utils/SimplifyCFG.cpp (+4-15) 
- (modified) llvm/test/Transforms/SimplifyCFG/X86/sink-common-code.ll (+6-15) 


``````````diff
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index ebdf760bda7f1a..3f680de98470ca 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2031,21 +2031,10 @@ static bool canSinkInstructions(
       return I->getOperand(OI) == I0->getOperand(OI);
     };
     if (!all_of(Insts, SameAsI0)) {
-      // Because SROA historically couldn't handle speculating stores of
-      // selects, we try not to sink loads, stores or lifetime markers of
-      // allocas when we'd have to create a PHI for the address operand.
-      // TODO: SROA supports speculation for loads and stores now -- remove
-      // this hack?
-      if (isa<StoreInst>(I0) && OI == 1 &&
-          any_of(Insts, [](const Instruction *I) {
-            return isa<AllocaInst>(I->getOperand(1)->stripPointerCasts());
-          }))
-        return false;
-      if (isa<LoadInst>(I0) && OI == 0 &&
-          any_of(Insts, [](const Instruction *I) {
-            return isa<AllocaInst>(I->getOperand(0)->stripPointerCasts());
-          }))
-        return false;
+      // SROA can't speculate lifetime markers of selects/phis, and the
+      // backend may handle such lifetimes incorrectly as well (#104776).
+      // Don't sink lifetimes if it would introduce a phi on the pointer
+      // argument.
       if (isLifeTimeMarker(I0) && OI == 1 &&
           any_of(Insts, [](const Instruction *I) {
             return isa<AllocaInst>(I->getOperand(1)->stripPointerCasts());
diff --git a/llvm/test/Transforms/SimplifyCFG/X86/sink-common-code.ll b/llvm/test/Transforms/SimplifyCFG/X86/sink-common-code.ll
index cd26d949836e68..39b1bec164b9e5 100644
--- a/llvm/test/Transforms/SimplifyCFG/X86/sink-common-code.ll
+++ b/llvm/test/Transforms/SimplifyCFG/X86/sink-common-code.ll
@@ -801,14 +801,8 @@ define i32 @test_pr30188(i1 zeroext %flag, i32 %x) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[Y:%.*]] = alloca i32, align 4
 ; CHECK-NEXT:    [[Z:%.*]] = alloca i32, align 4
-; CHECK-NEXT:    br i1 [[FLAG:%.*]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
-; CHECK:       if.then:
-; CHECK-NEXT:    store i32 [[X:%.*]], ptr [[Y]], align 4
-; CHECK-NEXT:    br label [[IF_END:%.*]]
-; CHECK:       if.else:
-; CHECK-NEXT:    store i32 [[X]], ptr [[Z]], align 4
-; CHECK-NEXT:    br label [[IF_END]]
-; CHECK:       if.end:
+; CHECK-NEXT:    [[Y_Z:%.*]] = select i1 [[FLAG:%.*]], ptr [[Y]], ptr [[Z]]
+; CHECK-NEXT:    store i32 [[X:%.*]], ptr [[Y_Z]], align 4
 ; CHECK-NEXT:    ret i32 1
 ;
 entry:
@@ -834,17 +828,14 @@ define i32 @test_pr30188a(i1 zeroext %flag, i32 %x) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[Y:%.*]] = alloca i32, align 4
 ; CHECK-NEXT:    [[Z:%.*]] = alloca i32, align 4
-; CHECK-NEXT:    br i1 [[FLAG:%.*]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
+; CHECK-NEXT:    br i1 [[FLAG:%.*]], label [[IF_THEN:%.*]], label [[IF_END:%.*]]
 ; CHECK:       if.then:
 ; CHECK-NEXT:    call void @g()
-; CHECK-NEXT:    [[ONE:%.*]] = load i32, ptr [[Y]], align 4
-; CHECK-NEXT:    br label [[IF_END:%.*]]
-; CHECK:       if.else:
-; CHECK-NEXT:    [[THREE:%.*]] = load i32, ptr [[Z]], align 4
 ; CHECK-NEXT:    br label [[IF_END]]
 ; CHECK:       if.end:
-; CHECK-NEXT:    [[THREE_SINK:%.*]] = phi i32 [ [[THREE]], [[IF_ELSE]] ], [ [[ONE]], [[IF_THEN]] ]
-; CHECK-NEXT:    [[FOUR:%.*]] = add i32 [[THREE_SINK]], 2
+; CHECK-NEXT:    [[Z_SINK:%.*]] = phi ptr [ [[Y]], [[IF_THEN]] ], [ [[Z]], [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[THREE:%.*]] = load i32, ptr [[Z_SINK]], align 4
+; CHECK-NEXT:    [[FOUR:%.*]] = add i32 [[THREE]], 2
 ; CHECK-NEXT:    store i32 [[FOUR]], ptr [[Y]], align 4
 ; CHECK-NEXT:    ret i32 1
 ;

``````````

</details>


https://github.com/llvm/llvm-project/pull/104788