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[LibCallSimplifier] try harder to fold memcmp with constant arguments

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Try to fold:
memcmp(X, C, ConstantLength) == 0 --> load X == *C

Without this change, we're unnecessarily checking the alignment of the constant data, 
so we miss the transform in the first 2 tests in the patch.

I noted this shortcoming of LibCallSimpifier in one of the recent CGP memcmp expansion 
patches. This doesn't help the example in:
https://bugs.llvm.org/show_bug.cgi?id=34032#c13
...directly, but it's worth short-circuiting more of these simple cases since we're 
already trying to do that.

The benefit of transforming to load+cmp is that existing IR analysis/transforms may
further simplify that code. For example, if the load of the variable is common to 
multiple memcmp calls, CSE can remove the duplicate instructions.

Differential Revision: https://reviews.llvm.org/D36922

llvm-svn: 311333
This commit is contained in:
Sanjay Patel
2017-08-21 13:55:49 +00:00
parent 9495f33e45
commit 7756edfa93
2 changed files with 92 additions and 11 deletions
Download Patch File Download Diff File Expand all files Collapse all files

38
llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
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@@ -18,6 +18,7 @@
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/ADT/Triple.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/OptimizationDiagnosticInfo.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
@@ -751,29 +752,44 @@ Value *LibCallSimplifier::optimizeMemCmp(CallInst *CI, IRBuilder<> &B) {
}
// memcmp(S1,S2,N/8)==0 -> (*(intN_t*)S1 != *(intN_t*)S2)==0
// TODO: The case where both inputs are constants does not need to be limited
// to legal integers or equality comparison. See block below this.
if (DL.isLegalInteger(Len * 8) && isOnlyUsedInZeroEqualityComparison(CI)) {
IntegerType *IntType = IntegerType::get(CI->getContext(), Len * 8);
unsigned PrefAlignment = DL.getPrefTypeAlignment(IntType);
if (getKnownAlignment(LHS, DL, CI) >= PrefAlignment &&
getKnownAlignment(RHS, DL, CI) >= PrefAlignment) {
// First, see if we can fold either argument to a constant.
Value *LHSV = nullptr;
if (auto *LHSC = dyn_cast<Constant>(LHS)) {
LHSC = ConstantExpr::getBitCast(LHSC, IntType->getPointerTo());
LHSV = ConstantFoldLoadFromConstPtr(LHSC, IntType, DL);
}
Value *RHSV = nullptr;
if (auto *RHSC = dyn_cast<Constant>(RHS)) {
RHSC = ConstantExpr::getBitCast(RHSC, IntType->getPointerTo());
RHSV = ConstantFoldLoadFromConstPtr(RHSC, IntType, DL);
}
// Don't generate unaligned loads. If either source is constant data,
// alignment doesn't matter for that source because there is no load.
if (!LHSV && getKnownAlignment(LHS, DL, CI) >= PrefAlignment) {
Type *LHSPtrTy =
IntType->getPointerTo(LHS->getType()->getPointerAddressSpace());
LHSV = B.CreateLoad(B.CreateBitCast(LHS, LHSPtrTy), "lhsv");
}
if (!RHSV && getKnownAlignment(RHS, DL, CI) >= PrefAlignment) {
Type *RHSPtrTy =
IntType->getPointerTo(RHS->getType()->getPointerAddressSpace());
Value *LHSV =
B.CreateLoad(B.CreateBitCast(LHS, LHSPtrTy, "lhsc"), "lhsv");
Value *RHSV =
B.CreateLoad(B.CreateBitCast(RHS, RHSPtrTy, "rhsc"), "rhsv");
return B.CreateZExt(B.CreateICmpNE(LHSV, RHSV), CI->getType(), "memcmp");
RHSV = B.CreateLoad(B.CreateBitCast(RHS, RHSPtrTy), "rhsv");
}
if (LHSV && RHSV)
return B.CreateZExt(B.CreateICmpNE(LHSV, RHSV), CI->getType(), "memcmp");
}
// Constant folding: memcmp(x, y, l) -> cnst (all arguments are constant)
// Constant folding: memcmp(x, y, Len) -> constant (all arguments are const).
// TODO: This is limited to i8 arrays.
StringRef LHSStr, RHSStr;
if (getConstantStringInfo(LHS, LHSStr) &&
getConstantStringInfo(RHS, RHSStr)) {