[LibCallSimplifier] try harder to fold memcmp with constant arguments (2nd try)
The 1st try was reverted because it could inf-loop by creating a dead instruction.
Fixed that to not happen and added a test case to verify.
Original commit message:
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: 311366
diff --git a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
index 8257dbc..37cfc2c 100644
--- a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -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 @@
}
// 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);
+ }
- Type *LHSPtrTy =
- IntType->getPointerTo(LHS->getType()->getPointerAddressSpace());
- 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");
-
+ // 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) &&
+ (RHSV || getKnownAlignment(RHS, DL, CI) >= PrefAlignment)) {
+ if (!LHSV) {
+ Type *LHSPtrTy =
+ IntType->getPointerTo(LHS->getType()->getPointerAddressSpace());
+ LHSV = B.CreateLoad(B.CreateBitCast(LHS, LHSPtrTy), "lhsv");
+ }
+ if (!RHSV) {
+ Type *RHSPtrTy =
+ IntType->getPointerTo(RHS->getType()->getPointerAddressSpace());
+ RHSV = B.CreateLoad(B.CreateBitCast(RHS, RHSPtrTy), "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)) {