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[LoopSimplify] Rebuild LCSSA for the inner loop after separating nested loops.
Summary: This hopefully fixes PR28825. The problem now was that a value from the original loop was used in a subloop, which became a sibling after separation. While a subloop doesn't need an lcssa phi node, a sibling does, and that's where we broke LCSSA. The most natural way to fix this now is to simply call formLCSSA on the original loop: it'll do what we've been doing before plus it'll cover situations described above. I think we don't need to run formLCSSARecursively here, and we have an assert to verify this (I've tried testing it on LLVM testsuite + SPECs). I'd be happy to be corrected here though. I also changed a run line in the test from '-lcssa -loop-unroll' to '-lcssa -loop-simplify -indvars', because it exercises LCSSA preservation to the same extent, but also makes less unrelated transformation on the CFG, which makes it easier to verify. Reviewers: chandlerc, sanjoy, silvas Subscribers: llvm-commits Differential Revision: https://reviews.llvm.org/D23288 llvm-svn: 278173
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@ -361,39 +361,11 @@ static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
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// Fix LCSSA form for L. Some values, which previously were only used inside
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// L, can now be used in NewOuter loop. We need to insert phi-nodes for them
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// in corresponding exit blocks.
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// We don't need to form LCSSA recursively, because there cannot be uses
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// inside a newly created loop of defs from inner loops as those would
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// already be a use of an LCSSA phi node.
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formLCSSA(*L, *DT, LI, SE);
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// Go through all instructions in OuterLoopBlocks and check if they are
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// using operands from the inner loop. In this case we'll need to fix LCSSA
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// for these instructions.
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SmallSetVector<Instruction *, 8> WorklistSet;
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for (BasicBlock *OuterBB: OuterLoopBlocks) {
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for (Instruction &I : *OuterBB) {
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for (Value *Op : I.operands()) {
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Instruction *OpI = dyn_cast<Instruction>(Op);
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if (!OpI || !L->contains(OpI))
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continue;
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WorklistSet.insert(OpI);
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}
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}
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}
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// We also need to check exit blocks of the outer loop - it might be using
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// values from what now became an inner loop.
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SmallVector<BasicBlock*, 8> ExitBlocks;
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NewOuter->getExitBlocks(ExitBlocks);
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for (BasicBlock *ExitBB: ExitBlocks) {
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for (Instruction &I : *ExitBB) {
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for (Value *Op : I.operands()) {
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Instruction *OpI = dyn_cast<Instruction>(Op);
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if (!OpI || !L->contains(OpI))
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continue;
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WorklistSet.insert(OpI);
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}
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}
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}
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SmallVector<Instruction *, 8> Worklist(WorklistSet.begin(),
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WorklistSet.end());
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formLCSSAForInstructions(Worklist, *DT, *LI);
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assert(NewOuter->isRecursivelyLCSSAForm(*DT) &&
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"LCSSA is broken after separating nested loops!");
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}
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@ -1,4 +1,4 @@
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; RUN: opt < %s -lcssa -loop-unroll -S | FileCheck %s
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; RUN: opt < %s -lcssa -loop-simplify -indvars -S | FileCheck %s
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target triple = "x86_64-unknown-linux-gnu"
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; PR28272, PR28825
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@ -106,3 +106,34 @@ bb_end:
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%x = getelementptr i32, i32* %b
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br label %bb_end
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}
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; When LoopSimplify separates nested loops, it might break LCSSA form: values
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; from the original loop might occur in a loop, which is now a sibling of the
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; original loop (before separating it was a subloop of the original loop, and
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; thus didn't require an lcssa phi nodes).
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; CHECK-LABEL: @foo4
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define void @foo4() {
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bb1:
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br label %bb2
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; CHECK: bb2.loopexit:
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bb2.loopexit: ; preds = %bb3
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%i.ph = phi i32 [ 0, %bb3 ]
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br label %bb2
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; CHECK: bb2.outer:
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; CHECK: bb2:
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bb2: ; preds = %bb2.loopexit, %bb2, %bb1
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%i = phi i32 [ 0, %bb1 ], [ %i, %bb2 ], [ %i.ph, %bb2.loopexit ]
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%x = load i32, i32* undef, align 8
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br i1 undef, label %bb2, label %bb3.preheader
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; CHECK: bb3.preheader:
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bb3.preheader: ; preds = %bb2
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; CHECK: %x.lcssa = phi i32 [ %x, %bb2 ]
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br label %bb3
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bb3: ; preds = %bb3.preheader, %bb3
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%y = add i32 2, %x
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br i1 true, label %bb2.loopexit, label %bb3
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}
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