forked from OSchip/llvm-project
[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
This commit is contained in:
Sanjay Patel
parent
d986545df6
commit
82ec872990
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@ -18,6 +18,7 @@
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#include "llvm/ADT/SmallString.h"
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#include "llvm/ADT/StringMap.h"
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#include "llvm/ADT/Triple.h"
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#include "llvm/Analysis/ConstantFolding.h"
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#include "llvm/Analysis/OptimizationDiagnosticInfo.h"
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#include "llvm/Analysis/TargetLibraryInfo.h"
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#include "llvm/Analysis/ValueTracking.h"
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@ -751,29 +752,44 @@ Value *LibCallSimplifier::optimizeMemCmp(CallInst *CI, IRBuilder<> &B) {
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}
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// memcmp(S1,S2,N/8)==0 -> (*(intN_t*)S1 != *(intN_t*)S2)==0
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// TODO: The case where both inputs are constants does not need to be limited
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// to legal integers or equality comparison. See block below this.
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if (DL.isLegalInteger(Len * 8) && isOnlyUsedInZeroEqualityComparison(CI)) {
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IntegerType *IntType = IntegerType::get(CI->getContext(), Len * 8);
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unsigned PrefAlignment = DL.getPrefTypeAlignment(IntType);
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if (getKnownAlignment(LHS, DL, CI) >= PrefAlignment &&
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getKnownAlignment(RHS, DL, CI) >= PrefAlignment) {
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Type *LHSPtrTy =
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IntType->getPointerTo(LHS->getType()->getPointerAddressSpace());
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Type *RHSPtrTy =
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IntType->getPointerTo(RHS->getType()->getPointerAddressSpace());
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Value *LHSV =
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B.CreateLoad(B.CreateBitCast(LHS, LHSPtrTy, "lhsc"), "lhsv");
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Value *RHSV =
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B.CreateLoad(B.CreateBitCast(RHS, RHSPtrTy, "rhsc"), "rhsv");
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// First, see if we can fold either argument to a constant.
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Value *LHSV = nullptr;
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if (auto *LHSC = dyn_cast<Constant>(LHS)) {
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LHSC = ConstantExpr::getBitCast(LHSC, IntType->getPointerTo());
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LHSV = ConstantFoldLoadFromConstPtr(LHSC, IntType, DL);
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}
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Value *RHSV = nullptr;
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if (auto *RHSC = dyn_cast<Constant>(RHS)) {
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RHSC = ConstantExpr::getBitCast(RHSC, IntType->getPointerTo());
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RHSV = ConstantFoldLoadFromConstPtr(RHSC, IntType, DL);
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}
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// Don't generate unaligned loads. If either source is constant data,
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// alignment doesn't matter for that source because there is no load.
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if ((LHSV || getKnownAlignment(LHS, DL, CI) >= PrefAlignment) &&
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(RHSV || getKnownAlignment(RHS, DL, CI) >= PrefAlignment)) {
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if (!LHSV) {
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Type *LHSPtrTy =
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IntType->getPointerTo(LHS->getType()->getPointerAddressSpace());
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LHSV = B.CreateLoad(B.CreateBitCast(LHS, LHSPtrTy), "lhsv");
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}
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if (!RHSV) {
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Type *RHSPtrTy =
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IntType->getPointerTo(RHS->getType()->getPointerAddressSpace());
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RHSV = B.CreateLoad(B.CreateBitCast(RHS, RHSPtrTy), "rhsv");
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}
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return B.CreateZExt(B.CreateICmpNE(LHSV, RHSV), CI->getType(), "memcmp");
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}
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}
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// Constant folding: memcmp(x, y, l) -> cnst (all arguments are constant)
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// Constant folding: memcmp(x, y, Len) -> constant (all arguments are const).
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// TODO: This is limited to i8 arrays.
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StringRef LHSStr, RHSStr;
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if (getConstantStringInfo(LHS, LHSStr) &&
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getConstantStringInfo(RHS, RHSStr)) {
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@ -3,31 +3,45 @@
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declare i32 @memcmp(i8*, i8*, i64)
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; TODO: The alignment of this constant does not matter. We constant fold the load.
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; The alignment of this constant does not matter. We constant fold the load.
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@charbuf = private unnamed_addr constant [4 x i8] [i8 0, i8 0, i8 0, i8 1], align 1
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define i1 @memcmp_4bytes_unaligned_constant_i8(i8* align 4 %x) {
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; ALL-LABEL: @memcmp_4bytes_unaligned_constant_i8(
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; ALL-NEXT: [[CALL:%.*]] = tail call i32 @memcmp(i8* %x, i8* getelementptr inbounds ([4 x i8], [4 x i8]* @charbuf, i64 0, i64 0), i64 4)
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; ALL-NEXT: [[CMPEQ0:%.*]] = icmp eq i32 [[CALL]], 0
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; ALL-NEXT: ret i1 [[CMPEQ0]]
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; LE-LABEL: @memcmp_4bytes_unaligned_constant_i8(
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; LE-NEXT: [[TMP1:%.*]] = bitcast i8* %x to i32*
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; LE-NEXT: [[LHSV:%.*]] = load i32, i32* [[TMP1]], align 4
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; LE-NEXT: [[TMP2:%.*]] = icmp eq i32 [[LHSV]], 16777216
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; LE-NEXT: ret i1 [[TMP2]]
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;
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; BE-LABEL: @memcmp_4bytes_unaligned_constant_i8(
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; BE-NEXT: [[TMP1:%.*]] = bitcast i8* %x to i32*
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; BE-NEXT: [[LHSV:%.*]] = load i32, i32* [[TMP1]], align 4
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; BE-NEXT: [[TMP2:%.*]] = icmp eq i32 [[LHSV]], 1
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; BE-NEXT: ret i1 [[TMP2]]
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;
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%call = tail call i32 @memcmp(i8* %x, i8* getelementptr inbounds ([4 x i8], [4 x i8]* @charbuf, i64 0, i64 0), i64 4)
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%cmpeq0 = icmp eq i32 %call, 0
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ret i1 %cmpeq0
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}
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; TODO: We still don't care about alignment of the constant. We are not limited to constant folding only i8 arrays.
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; We still don't care about alignment of the constant. We are not limited to constant folding only i8 arrays.
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; It doesn't matter if the constant operand is the first operand to the memcmp.
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@intbuf_unaligned = private unnamed_addr constant [4 x i16] [i16 1, i16 2, i16 3, i16 4], align 1
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define i1 @memcmp_4bytes_unaligned_constant_i16(i8* align 4 %x) {
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; ALL-LABEL: @memcmp_4bytes_unaligned_constant_i16(
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; ALL-NEXT: [[CALL:%.*]] = tail call i32 @memcmp(i8* bitcast ([4 x i16]* @intbuf_unaligned to i8*), i8* %x, i64 4)
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; ALL-NEXT: [[CMPEQ0:%.*]] = icmp eq i32 [[CALL]], 0
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; ALL-NEXT: ret i1 [[CMPEQ0]]
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; LE-LABEL: @memcmp_4bytes_unaligned_constant_i16(
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; LE-NEXT: [[TMP1:%.*]] = bitcast i8* %x to i32*
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; LE-NEXT: [[RHSV:%.*]] = load i32, i32* [[TMP1]], align 4
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; LE-NEXT: [[TMP2:%.*]] = icmp eq i32 [[RHSV]], 131073
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; LE-NEXT: ret i1 [[TMP2]]
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;
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; BE-LABEL: @memcmp_4bytes_unaligned_constant_i16(
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; BE-NEXT: [[TMP1:%.*]] = bitcast i8* %x to i32*
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; BE-NEXT: [[RHSV:%.*]] = load i32, i32* [[TMP1]], align 4
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; BE-NEXT: [[TMP2:%.*]] = icmp eq i32 [[RHSV]], 65538
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; BE-NEXT: ret i1 [[TMP2]]
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;
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%call = tail call i32 @memcmp(i8* bitcast (i16* getelementptr inbounds ([4 x i16], [4 x i16]* @intbuf_unaligned, i64 0, i64 0) to i8*), i8* %x, i64 4)
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%cmpeq0 = icmp eq i32 %call, 0
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@ -49,3 +63,18 @@ define i1 @memcmp_3bytes_aligned_constant_i32(i8* align 4 %x) {
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ret i1 %cmpeq0
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}
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; A sloppy implementation would infinite loop by recreating the unused instructions.
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define i1 @memcmp_4bytes_one_unaligned_i8(i8* align 4 %x, i8* align 1 %y) {
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; ALL-LABEL: @memcmp_4bytes_one_unaligned_i8(
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; ALL-NEXT: [[CALL:%.*]] = tail call i32 @memcmp(i8* %x, i8* %y, i64 4)
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; ALL-NEXT: [[CMPEQ0:%.*]] = icmp eq i32 [[CALL]], 0
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; ALL-NEXT: ret i1 [[CMPEQ0]]
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;
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%bc = bitcast i8* %x to i32*
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%lhsv = load i32, i32* %bc
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%call = tail call i32 @memcmp(i8* %x, i8* %y, i64 4)
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%cmpeq0 = icmp eq i32 %call, 0
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ret i1 %cmpeq0
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}
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