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Fix a performance regression when combining to smaller types in icmp (shl %v, C1), C2 :

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Only combine when the shl is only used by the icmp

llvm-svn: 176950
This commit is contained in:
Arnaud A. de Grandmaison 2013-03-13 14:40:37 +00:00
parent 10d47de154
commit 0810447275
1 changed files with 4 additions and 3 deletions
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7
lib/Transforms/InstCombine/InstCombineCompares.cpp
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@ -1333,13 +1333,14 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
}
// Transform (icmp pred iM (shl iM %v, N), CI)
// -> (icmp pred i(M-N) (trunc %v iM to i(N-N)), (trunc (CI>>N))
// Transform the shl to a trunc if (trunc (CI>>N)) has no loss.
// -> (icmp pred i(M-N) (trunc %v iM to i(M-N)), (trunc (CI>>N))
// Transform the shl to a trunc if (trunc (CI>>N)) has no loss and M-N.
// This enables to get rid of the shift in favor of a trunc which can be
// free on the target. It has the additional benefit of comparing to a
// smaller constant, which will be target friendly.
unsigned Amt = ShAmt->getLimitedValue(TypeBits-1);
if (Amt != 0 && RHSV.countTrailingZeros() >= Amt) {
if (LHSI->hasOneUse() &&
Amt != 0 && RHSV.countTrailingZeros() >= Amt) {
Type *NTy = IntegerType::get(ICI.getContext(), TypeBits - Amt);
Constant *NCI = ConstantExpr::getTrunc(
ConstantExpr::getAShr(RHS,