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[LICM] Compute a must execute property for the prefix of the header as we go

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Computing this property within the existing walk ensures that the cost is linear with the size of the block. If we did this from within isGuaranteedToExecute, it would be quadratic without some very fancy caching.

This allows us to reliably catch a hoistable instruction within a header which may throw at some point *after* our hoistable instruction. It doesn't do anything for non-header cases, but given how common single block loops are, this seems very worthwhile.

llvm-svn: 331557
This commit is contained in:
Philip Reames 2018-05-04 21:35:00 +00:00
parent 4d27156649
commit 24db79c9c9
2 changed files with 34 additions and 3 deletions
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17
lib/Transforms/Scalar/LICM.cpp
View File

@ -449,6 +449,11 @@ bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
if (inSubLoop(BB, CurLoop, LI))
continue;
// Keep track of whether the prefix of instructions visited so far are such
// that the next instruction visited is guaranteed to execute if the loop
// is entered.
bool IsMustExecute = CurLoop->getHeader() == BB;
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
Instruction &I = *II++;
// Try constant folding this instruction. If all the operands are
@ -496,10 +501,16 @@ bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
//
if (CurLoop->hasLoopInvariantOperands(&I) &&
canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, SafetyInfo, ORE) &&
isSafeToExecuteUnconditionally(
I, DT, CurLoop, SafetyInfo, ORE,
CurLoop->getLoopPreheader()->getTerminator()))
(IsMustExecute ||
isSafeToExecuteUnconditionally(
I, DT, CurLoop, SafetyInfo, ORE,
CurLoop->getLoopPreheader()->getTerminator()))) {
Changed |= hoist(I, DT, CurLoop, SafetyInfo, ORE);
continue;
}
if (IsMustExecute)
IsMustExecute = isGuaranteedToTransferExecutionToSuccessor(&I);
}
}

20
test/Transforms/LICM/preheader-safe.ll
View File

@ -55,7 +55,27 @@ loop: ; preds = %entry, %for.inc
br label %loop
}
; Similiar to the above, but the hoistable instruction (%y in this case)
; happens not to be the first instruction in the block.
define void @throw_header_after_nonfirst(i64* %xp, i64* %yp, i1* %cond) {
; CHECK-LABEL: throw_header_after_nonfirst
; CHECK: %y = load i64, i64* %yp
; CHECK-LABEL: loop
; CHECK: %x = load i64, i64* %gep
; CHECK: %div = udiv i64 %x, %y
; CHECK: call void @use(i64 %div)
entry:
br label %loop
loop: ; preds = %entry, %for.inc
%iv = phi i64 [0, %entry], [%div, %loop]
%gep = getelementptr i64, i64* %xp, i64 %iv
%x = load i64, i64* %gep
%y = load i64, i64* %yp
%div = udiv i64 %x, %y
call void @use(i64 %div) readonly
br label %loop
}
; Negative test
define void @throw_header_before(i64 %x, i64 %y, i1* %cond) {