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InstCombine: Combine select sequences into a single select

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Normalize
select(C0, select(C1, a, b), b) -> select((C0 & C1), a, b)
select(C0, a, select(C1, a, b)) -> select((C0 | C1), a, b)

This normal form may enable further combines on the And/Or and shortens
paths for the values. Many targets prefer the other but can go back
easily in CodeGen.

Differential Revision: http://reviews.llvm.org/D7399

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228409 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Matthias Braun
2015-02-06 17:49:36 +00:00
parent e81cc348ac
commit 2f2dec87fb
2 changed files with 46 additions and 0 deletions
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18
lib/Transforms/InstCombine/InstCombineSelect.cpp
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@@ -1178,20 +1178,38 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
return NV;
if (SelectInst *TrueSI = dyn_cast<SelectInst>(TrueVal)) {
// select(C, select(C, a, b), c) -> select(C, a, c)
if (TrueSI->getCondition() == CondVal) {
if (SI.getTrueValue() == TrueSI->getTrueValue())
return nullptr;
SI.setOperand(1, TrueSI->getTrueValue());
return &SI;
}
// select(C0, select(C1, a, b), b) -> select(C0&C1, a, b)
// We choose this as normal form to enable folding on the And and shortening
// paths for the values (this helps GetUnderlyingObjects() for example).
if (TrueSI->getFalseValue() == FalseVal && TrueSI->hasOneUse()) {
Value *And = Builder->CreateAnd(CondVal, TrueSI->getCondition());
SI.setOperand(0, And);
SI.setOperand(1, TrueSI->getTrueValue());
return &SI;
}
}
if (SelectInst *FalseSI = dyn_cast<SelectInst>(FalseVal)) {
// select(C, a, select(C, b, c)) -> select(C, a, c)
if (FalseSI->getCondition() == CondVal) {
if (SI.getFalseValue() == FalseSI->getFalseValue())
return nullptr;
SI.setOperand(2, FalseSI->getFalseValue());
return &SI;
}
// select(C0, a, select(C1, a, b)) -> select(C0|C1, a, b)
if (FalseSI->getTrueValue() == TrueVal && FalseSI->hasOneUse()) {
Value *Or = Builder->CreateOr(CondVal, FalseSI->getCondition());
SI.setOperand(0, Or);
SI.setOperand(2, FalseSI->getFalseValue());
return &SI;
}
}
if (BinaryOperator::isNot(CondVal)) {