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ValueTracking: Make isBytewiseValue simpler and more powerful at the same time.

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Turns out there is a simpler way of checking that all bytes in a word are equal
than binary decomposition.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228503 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer
2015-02-07 19:29:02 +00:00
parent 2dd5f23a1d
commit a54b82a9fe
2 changed files with 23 additions and 18 deletions
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26
lib/Analysis/ValueTracking.cpp
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@ -2121,26 +2121,16 @@ Value *llvm::isBytewiseValue(Value *V) {
// Don't handle long double formats, which have strange constraints.
}
// We can handle constant integers that are power of two in size and a
// multiple of 8 bits.
// We can handle constant integers that are multiple of 8 bits.
if (ConstantInt *CI = dyn_cast<ConstantInt>(V)) {
unsigned Width = CI->getBitWidth();
if (isPowerOf2_32(Width) && Width > 8) {
// We can handle this value if the recursive binary decomposition is the
// same at all levels.
APInt Val = CI->getValue();
APInt Val2;
while (Val.getBitWidth() != 8) {
unsigned NextWidth = Val.getBitWidth()/2;
Val2 = Val.lshr(NextWidth);
Val2 = Val2.trunc(Val.getBitWidth()/2);
Val = Val.trunc(Val.getBitWidth()/2);
if (CI->getBitWidth() % 8 == 0) {
assert(CI->getBitWidth() > 8 && "8 bits should be handled above!");
// If the top/bottom halves aren't the same, reject it.
if (Val != Val2)
return nullptr;
}
return ConstantInt::get(V->getContext(), Val);
// We can check that all bytes of an integer are equal by making use of a
// little trick: rotate by 8 and check if it's still the same value.
if (CI->getValue() != CI->getValue().rotl(8))
return nullptr;
return ConstantInt::get(V->getContext(), CI->getValue().trunc(8));
}
}