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[LoopDeletion] Revert 3af8a11 and add test coverage for breakage

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This reverts 3af8a11 because I'd used an upper bound where an lower bound was required.  The included reduced test case demonstrates the issue.
This commit is contained in:
Philip Reames
2022-01-17 11:39:19 -08:00
parent f40a579bea
commit 523573e90d
2 changed files with 58 additions and 18 deletions
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29
llvm/lib/Transforms/Scalar/LoopDeletion.cpp
View File

@@ -407,25 +407,18 @@ breakBackedgeIfNotTaken(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
if (!L->getLoopLatch())
return LoopDeletionResult::Unmodified;
auto *BTC = SE.getSymbolicMaxBackedgeTakenCount(L);
if (BTC->isZero()) {
// SCEV knows this backedge isn't taken!
breakLoopBackedge(L, DT, SE, LI, MSSA);
++NumBackedgesBroken;
return LoopDeletionResult::Deleted;
}
// If SCEV leaves open the possibility of a zero trip count, see if
// symbolically evaluating the first iteration lets us prove the backedge
// unreachable.
if (isa<SCEVCouldNotCompute>(BTC) || !SE.isKnownNonZero(BTC))
if (canProveExitOnFirstIteration(L, DT, LI)) {
breakLoopBackedge(L, DT, SE, LI, MSSA);
++NumBackedgesBroken;
return LoopDeletionResult::Deleted;
auto *BTCMax = SE.getConstantMaxBackedgeTakenCount(L);
if (!BTCMax->isZero()) {
auto *BTC = SE.getBackedgeTakenCount(L);
if (!BTC->isZero()) {
if (!isa<SCEVCouldNotCompute>(BTC) && SE.isKnownNonZero(BTC))
return LoopDeletionResult::Unmodified;
if (!canProveExitOnFirstIteration(L, DT, LI))
return LoopDeletionResult::Unmodified;
}
return LoopDeletionResult::Unmodified;
}
breakLoopBackedge(L, DT, SE, LI, MSSA);
return LoopDeletionResult::Deleted;
}
/// Remove a loop if it is dead.

47
llvm/test/Transforms/LoopDeletion/D108848-regression.ll Normal file
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@@ -0,0 +1,47 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt -loop-deletion -S < %s | FileCheck %s
@a = global i1 false, align 4
define i32 @main() {
; CHECK-LABEL: @main(
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[FOR_BODY:%.*]]
; CHECK: for.body:
; CHECK-NEXT: [[D_015:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ]
; CHECK-NEXT: call void @foo()
; CHECK-NEXT: [[DOTB:%.*]] = load i1, i1* @a, align 4
; CHECK-NEXT: [[CONV1:%.*]] = select i1 [[DOTB]], i32 -128, i32 1
; CHECK-NEXT: [[CMP2_NOT:%.*]] = icmp ugt i32 [[CONV1]], [[D_015]]
; CHECK-NEXT: br i1 [[CMP2_NOT]], label [[CLEANUP:%.*]], label [[FOR_INC8:%.*]]
; CHECK: for.inc8:
; CHECK-NEXT: [[INC9:%.*]] = add nuw nsw i32 [[D_015]], 1
; CHECK-NEXT: [[EXITCOND_NOT:%.*]] = icmp eq i32 [[INC9]], 19
; CHECK-NEXT: br label [[CLEANUP]]
; CHECK: cleanup:
; CHECK-NEXT: store i1 true, i1* @a, align 4
; CHECK-NEXT: ret i32 0
;
entry:
br label %for.body
for.body:
%d.015 = phi i32 [ 0, %entry ], [ %inc9, %for.inc8 ]
call void @foo()
%.b = load i1, i1* @a, align 4
%conv1 = select i1 %.b, i32 -128, i32 1
%cmp2.not = icmp ugt i32 %conv1, %d.015
br i1 %cmp2.not, label %cleanup, label %for.inc8
for.inc8:
%inc9 = add nuw nsw i32 %d.015, 1
%exitcond.not = icmp eq i32 %inc9, 19
br i1 %exitcond.not, label %cleanup, label %for.body
cleanup:
store i1 true, i1* @a, align 4
ret i32 0
}
declare void @foo()