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[SimplifyCFG] FoldBranchToCommonDest(): properly handle same-block external uses (PR49510/PR49689) 

We clone bonus instructions to the end of the predecessor block,
and then use `SSAUpdater::RewriteUseAfterInsertions()`.
But that only deals with the cases where the use-to-be-rewritten
are either in different block from the def, or come after the def.

But in some loop cases, the external use may be in the beginning of
predecessor block, before the newly cloned bonus instruction.
`SSAUpdater::RewriteUseAfterInsertions()` does not deal with that.
Notably, the external use can't happen to be both in the same block
and *after* the newly-cloned instruction, because of the fold preconditions.

To properly handle these cases, when the use is in the same block,
we should instead use `SSAUpdater::RewriteUse()`.
TBN, they do the same thing for PHI users.

Fixes https://bugs.llvm.org/show_bug.cgi?id=49510
Likely Fixes https://bugs.llvm.org/show_bug.cgi?id=49689
This commit is contained in:
Roman Lebedev 2021-03-23 17:33:40 +03:00
parent bc6b139392
commit 514bc01ca3
No known key found for this signature in database
GPG Key ID: 083C3EBB4A1689E0
2 changed files with 41 additions and 2 deletions
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9
llvm/lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -1097,8 +1097,13 @@ static void CloneInstructionsIntoPredecessorBlockAndUpdateSSAUses(
(NewBonusInst->getName() + ".merge").str());
SSAUpdate.AddAvailableValue(BB, &BonusInst);
SSAUpdate.AddAvailableValue(PredBlock, NewBonusInst);
for (Use &U : make_early_inc_range(BonusInst.uses()))
SSAUpdate.RewriteUseAfterInsertions(U);
for (Use &U : make_early_inc_range(BonusInst.uses())) {
auto *UI = cast<Instruction>(U.getUser());
if (UI->getParent() != PredBlock)
SSAUpdate.RewriteUseAfterInsertions(U);
else // Use is in the same block as, and comes before, NewBonusInst.
SSAUpdate.RewriteUse(U);
}
}
}

34
llvm/test/Transforms/SimplifyCFG/fold-branch-to-common-dest.ll
View File

@ -894,3 +894,37 @@ if.end7:
cleanup:
unreachable
}
@global_pr49510 = external global i16, align 1
define void @pr49510() {
; CHECK-LABEL: @pr49510(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[DOTOLD:%.*]] = load i16, i16* @global_pr49510, align 1
; CHECK-NEXT: [[TOBOOL_OLD:%.*]] = icmp ne i16 [[DOTOLD]], 0
; CHECK-NEXT: br i1 [[TOBOOL_OLD]], label [[LAND_RHS:%.*]], label [[FOR_END:%.*]]
; CHECK: land.rhs:
; CHECK-NEXT: [[DOTMERGE:%.*]] = phi i16 [ [[TMP0:%.*]], [[LAND_RHS]] ], [ [[DOTOLD]], [[ENTRY:%.*]] ]
; CHECK-NEXT: [[CMP:%.*]] = icmp slt i16 [[DOTMERGE]], 0
; CHECK-NEXT: [[TMP0]] = load i16, i16* @global_pr49510, align 1
; CHECK-NEXT: [[TOBOOL:%.*]] = icmp ne i16 [[TMP0]], 0
; CHECK-NEXT: [[OR_COND:%.*]] = select i1 [[CMP]], i1 [[TOBOOL]], i1 false
; CHECK-NEXT: br i1 [[OR_COND]], label [[LAND_RHS]], label [[FOR_END]]
; CHECK: for.end:
; CHECK-NEXT: ret void
;
entry:
br label %for.cond
for.cond:
%0 = load i16, i16* @global_pr49510, align 1
%tobool = icmp ne i16 %0, 0
br i1 %tobool, label %land.rhs, label %for.end
land.rhs:
%cmp = icmp slt i16 %0, 0
br i1 %cmp, label %for.cond, label %for.end
for.end:
ret void
}