[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 2017-08-21 19:13:14 +00:00
parent d986545df6
commit 82ec872990
2 changed files with 69 additions and 24 deletions

View File

@ -18,6 +18,7 @@
#include "llvm/ADT/SmallString.h" #include "llvm/ADT/SmallString.h"
#include "llvm/ADT/StringMap.h" #include "llvm/ADT/StringMap.h"
#include "llvm/ADT/Triple.h" #include "llvm/ADT/Triple.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/OptimizationDiagnosticInfo.h" #include "llvm/Analysis/OptimizationDiagnosticInfo.h"
#include "llvm/Analysis/TargetLibraryInfo.h" #include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.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 // 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)) { if (DL.isLegalInteger(Len * 8) && isOnlyUsedInZeroEqualityComparison(CI)) {
IntegerType *IntType = IntegerType::get(CI->getContext(), Len * 8); IntegerType *IntType = IntegerType::get(CI->getContext(), Len * 8);
unsigned PrefAlignment = DL.getPrefTypeAlignment(IntType); unsigned PrefAlignment = DL.getPrefTypeAlignment(IntType);
if (getKnownAlignment(LHS, DL, CI) >= PrefAlignment && // First, see if we can fold either argument to a constant.
getKnownAlignment(RHS, DL, CI) >= PrefAlignment) { Value *LHSV = nullptr;
if (auto *LHSC = dyn_cast<Constant>(LHS)) {
Type *LHSPtrTy = LHSC = ConstantExpr::getBitCast(LHSC, IntType->getPointerTo());
IntType->getPointerTo(LHS->getType()->getPointerAddressSpace()); LHSV = ConstantFoldLoadFromConstPtr(LHSC, IntType, DL);
Type *RHSPtrTy = }
IntType->getPointerTo(RHS->getType()->getPointerAddressSpace()); Value *RHSV = nullptr;
if (auto *RHSC = dyn_cast<Constant>(RHS)) {
Value *LHSV = RHSC = ConstantExpr::getBitCast(RHSC, IntType->getPointerTo());
B.CreateLoad(B.CreateBitCast(LHS, LHSPtrTy, "lhsc"), "lhsv"); RHSV = ConstantFoldLoadFromConstPtr(RHSC, IntType, DL);
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"); 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; StringRef LHSStr, RHSStr;
if (getConstantStringInfo(LHS, LHSStr) && if (getConstantStringInfo(LHS, LHSStr) &&
getConstantStringInfo(RHS, RHSStr)) { getConstantStringInfo(RHS, RHSStr)) {

View File

@ -3,31 +3,45 @@
declare i32 @memcmp(i8*, i8*, i64) declare i32 @memcmp(i8*, i8*, i64)
; TODO: The alignment of this constant does not matter. We constant fold the load. ; The alignment of this constant does not matter. We constant fold the load.
@charbuf = private unnamed_addr constant [4 x i8] [i8 0, i8 0, i8 0, i8 1], align 1 @charbuf = private unnamed_addr constant [4 x i8] [i8 0, i8 0, i8 0, i8 1], align 1
define i1 @memcmp_4bytes_unaligned_constant_i8(i8* align 4 %x) { define i1 @memcmp_4bytes_unaligned_constant_i8(i8* align 4 %x) {
; ALL-LABEL: @memcmp_4bytes_unaligned_constant_i8( ; LE-LABEL: @memcmp_4bytes_unaligned_constant_i8(
; 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) ; LE-NEXT: [[TMP1:%.*]] = bitcast i8* %x to i32*
; ALL-NEXT: [[CMPEQ0:%.*]] = icmp eq i32 [[CALL]], 0 ; LE-NEXT: [[LHSV:%.*]] = load i32, i32* [[TMP1]], align 4
; ALL-NEXT: ret i1 [[CMPEQ0]] ; LE-NEXT: [[TMP2:%.*]] = icmp eq i32 [[LHSV]], 16777216
; LE-NEXT: ret i1 [[TMP2]]
;
; BE-LABEL: @memcmp_4bytes_unaligned_constant_i8(
; BE-NEXT: [[TMP1:%.*]] = bitcast i8* %x to i32*
; BE-NEXT: [[LHSV:%.*]] = load i32, i32* [[TMP1]], align 4
; BE-NEXT: [[TMP2:%.*]] = icmp eq i32 [[LHSV]], 1
; BE-NEXT: ret i1 [[TMP2]]
; ;
%call = tail call i32 @memcmp(i8* %x, i8* getelementptr inbounds ([4 x i8], [4 x i8]* @charbuf, i64 0, i64 0), i64 4) %call = tail call i32 @memcmp(i8* %x, i8* getelementptr inbounds ([4 x i8], [4 x i8]* @charbuf, i64 0, i64 0), i64 4)
%cmpeq0 = icmp eq i32 %call, 0 %cmpeq0 = icmp eq i32 %call, 0
ret i1 %cmpeq0 ret i1 %cmpeq0
} }
; TODO: We still don't care about alignment of the constant. We are not limited to constant folding only i8 arrays. ; We still don't care about alignment of the constant. We are not limited to constant folding only i8 arrays.
; It doesn't matter if the constant operand is the first operand to the memcmp. ; It doesn't matter if the constant operand is the first operand to the memcmp.
@intbuf_unaligned = private unnamed_addr constant [4 x i16] [i16 1, i16 2, i16 3, i16 4], align 1 @intbuf_unaligned = private unnamed_addr constant [4 x i16] [i16 1, i16 2, i16 3, i16 4], align 1
define i1 @memcmp_4bytes_unaligned_constant_i16(i8* align 4 %x) { define i1 @memcmp_4bytes_unaligned_constant_i16(i8* align 4 %x) {
; ALL-LABEL: @memcmp_4bytes_unaligned_constant_i16( ; LE-LABEL: @memcmp_4bytes_unaligned_constant_i16(
; ALL-NEXT: [[CALL:%.*]] = tail call i32 @memcmp(i8* bitcast ([4 x i16]* @intbuf_unaligned to i8*), i8* %x, i64 4) ; LE-NEXT: [[TMP1:%.*]] = bitcast i8* %x to i32*
; ALL-NEXT: [[CMPEQ0:%.*]] = icmp eq i32 [[CALL]], 0 ; LE-NEXT: [[RHSV:%.*]] = load i32, i32* [[TMP1]], align 4
; ALL-NEXT: ret i1 [[CMPEQ0]] ; LE-NEXT: [[TMP2:%.*]] = icmp eq i32 [[RHSV]], 131073
; LE-NEXT: ret i1 [[TMP2]]
;
; BE-LABEL: @memcmp_4bytes_unaligned_constant_i16(
; BE-NEXT: [[TMP1:%.*]] = bitcast i8* %x to i32*
; BE-NEXT: [[RHSV:%.*]] = load i32, i32* [[TMP1]], align 4
; BE-NEXT: [[TMP2:%.*]] = icmp eq i32 [[RHSV]], 65538
; BE-NEXT: ret i1 [[TMP2]]
; ;
%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) %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)
%cmpeq0 = icmp eq i32 %call, 0 %cmpeq0 = icmp eq i32 %call, 0
@ -49,3 +63,18 @@ define i1 @memcmp_3bytes_aligned_constant_i32(i8* align 4 %x) {
ret i1 %cmpeq0 ret i1 %cmpeq0
} }
; A sloppy implementation would infinite loop by recreating the unused instructions.
define i1 @memcmp_4bytes_one_unaligned_i8(i8* align 4 %x, i8* align 1 %y) {
; ALL-LABEL: @memcmp_4bytes_one_unaligned_i8(
; ALL-NEXT: [[CALL:%.*]] = tail call i32 @memcmp(i8* %x, i8* %y, i64 4)
; ALL-NEXT: [[CMPEQ0:%.*]] = icmp eq i32 [[CALL]], 0
; ALL-NEXT: ret i1 [[CMPEQ0]]
;
%bc = bitcast i8* %x to i32*
%lhsv = load i32, i32* %bc
%call = tail call i32 @memcmp(i8* %x, i8* %y, i64 4)
%cmpeq0 = icmp eq i32 %call, 0
ret i1 %cmpeq0
}