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fix two significant issues with FoldTwoEntryPHINode:

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first, it can kick in on blocks whose conditions have been
folded to a constant, even though one of the edges will be
trivially folded.

second, it doesn't clean up the "if diamond" that it just 
eliminated away.  This is a problem because other simplifycfg
xforms kick in depending on the order of block visitation,
causing pointless work.

llvm-svn: 121762
This commit is contained in:
Chris Lattner 2010-12-14 08:01:53 +00:00
parent 362f9a82c0
commit 22d4dc5a4d
2 changed files with 16 additions and 3 deletions
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12
lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -1148,7 +1148,10 @@ static bool FoldTwoEntryPHINode(PHINode *PN, const TargetData *TD) {
BasicBlock *BB = PN->getParent();
BasicBlock *IfTrue, *IfFalse;
Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse);
if (!IfCond) return false;
if (!IfCond ||
// Don't bother if the branch will be constant folded trivially.
isa<ConstantInt>(IfCond))
return false;
// Okay, we found that we can merge this two-entry phi node into a select.
// Doing so would require us to fold *all* two entry phi nodes in this block.
@ -1243,6 +1246,13 @@ static bool FoldTwoEntryPHINode(PHINode *PN, const TargetData *TD) {
NV->takeName(PN);
PN->eraseFromParent();
}
// At this point, IfBlock1 and IfBlock2 are both empty, so our if statement
// has been flattened. Change DomBlock to jump directly to our new block to
// avoid other simplifycfg's kicking in on the diamond.
TerminatorInst *OldTI = DomBlock->getTerminator();
BranchInst::Create(BB, OldTI);
OldTI->eraseFromParent();
return true;
}

7
test/Transforms/SimplifyCFG/PhiBlockMerge.ll
View File

@ -1,12 +1,14 @@
; Test merging of blocks that only have PHI nodes in them
;
; RUN: opt < %s -simplifycfg -S | not grep N:
; RUN: opt < %s -simplifycfg -S | FileCheck %s
;
define i32 @test(i1 %a, i1 %b) {
; <label>:0
; CHECK: br i1 %a
br i1 %a, label %M, label %O
O: ; preds = %0
; CHECK: select i1 %b, i32 0, i32 1
; CHECK-NOT: phi
br i1 %b, label %N, label %Q
Q: ; preds = %O
br label %N
@ -15,6 +17,7 @@ N: ; preds = %Q, %O
%Wp = phi i32 [ 0, %O ], [ 1, %Q ] ; <i32> [#uses=1]
br label %M
M: ; preds = %N, %0
; CHECK: %W = phi i32
%W = phi i32 [ %Wp, %N ], [ 2, %0 ] ; <i32> [#uses=1]
%R = add i32 %W, 1 ; <i32> [#uses=1]
ret i32 %R