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[SelectionDAG] Allow the user to specify a memeq function.

Summary:
Right now, when we encounter a string equality check,
e.g. `if (memcmp(a, b, s) == 0)`, we try to expand to a comparison if `s` is a
small compile-time constant, and fall back on calling `memcmp()` else.

This is sub-optimal because memcmp has to compute much more than
equality.

This patch replaces `memcmp(a, b, s) == 0` by `bcmp(a, b, s) == 0` on platforms
that support `bcmp`.

`bcmp` can be made much more efficient than `memcmp` because equality
compare is trivially parallel while lexicographic ordering has a chain
dependency.

Subscribers: fedor.sergeev, jyknight, ckennelly, gchatelet, llvm-commits

Differential Revision: https://reviews.llvm.org/D56593

llvm-svn: 355672
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legrosbuffle committed Mar 8, 2019
1 parent 1a98dc1 commit 8e16d73346f8091461319a7dfc4ddd18eedcff13
@@ -40,6 +40,11 @@ Non-comprehensive list of changes in this release
functionality, or simply have a lot to talk about), see the `NOTE` below
for adding a new subsection.
* The optimizer will now convert calls to memcmp into a calls to bcmp in some
circumstances. Users who are building freestanding code (not depending on the
platform's libc) without specifying -ffreestanding may need to either pass
-fno-builtin-bcmp, or provide a bcmp function.

.. NOTE
If you would like to document a larger change, then you can add a
subsection about it right here. You can copy the following boilerplate
@@ -92,6 +92,10 @@ namespace llvm {
Value *emitMemCmp(Value *Ptr1, Value *Ptr2, Value *Len, IRBuilder<> &B,
const DataLayout &DL, const TargetLibraryInfo *TLI);

/// Emit a call to the bcmp function.
Value *emitBCmp(Value *Ptr1, Value *Ptr2, Value *Len, IRBuilder<> &B,
const DataLayout &DL, const TargetLibraryInfo *TLI);

/// Emit a call to the unary function named 'Name' (e.g. 'floor'). This
/// function is known to take a single of type matching 'Op' and returns one
/// value with the same type. If 'Op' is a long double, 'l' is added as the
@@ -49,6 +49,16 @@ static bool hasSinCosPiStret(const Triple &T) {
return true;
}

static bool hasBcmp(const Triple &TT) {
// Posix removed support from bcmp() in 2001, but the glibc and several
// implementations of the libc still have it.
if (TT.isOSLinux())
return TT.isGNUEnvironment() || TT.isMusl();
// Both NetBSD and OpenBSD are planning to remove the function. Windows does
// not have it.
return TT.isOSFreeBSD() || TT.isOSSolaris() || TT.isOSDarwin();
}

/// Initialize the set of available library functions based on the specified
/// target triple. This should be carefully written so that a missing target
/// triple gets a sane set of defaults.
@@ -141,6 +151,9 @@ static void initialize(TargetLibraryInfoImpl &TLI, const Triple &T,
TLI.setUnavailable(LibFunc_sincospif_stret);
}

if (!hasBcmp(T))
TLI.setUnavailable(LibFunc_bcmp);

if (T.isMacOSX() && T.getArch() == Triple::x86 &&
!T.isMacOSXVersionLT(10, 7)) {
// x86-32 OSX has a scheme where fwrite and fputs (and some other functions
@@ -930,28 +930,41 @@ Value *llvm::emitMemChr(Value *Ptr, Value *Val, Value *Len, IRBuilder<> &B,
return CI;
}

Value *llvm::emitMemCmp(Value *Ptr1, Value *Ptr2, Value *Len, IRBuilder<> &B,
const DataLayout &DL, const TargetLibraryInfo *TLI) {
if (!TLI->has(LibFunc_memcmp))
// Common code for memcmp() and bcmp(), which have the exact same properties,
// just a slight difference in semantics.
static Value *emitMemCmpOrBcmp(llvm::LibFunc libfunc, Value *Ptr1, Value *Ptr2,
Value *Len, IRBuilder<> &B, const DataLayout &DL,
const TargetLibraryInfo *TLI) {
if (!TLI->has(libfunc))
return nullptr;

Module *M = B.GetInsertBlock()->getModule();
StringRef MemCmpName = TLI->getName(LibFunc_memcmp);
StringRef CmpFnName = TLI->getName(libfunc);
LLVMContext &Context = B.GetInsertBlock()->getContext();
FunctionCallee MemCmp =
M->getOrInsertFunction(MemCmpName, B.getInt32Ty(), B.getInt8PtrTy(),
FunctionCallee CmpFn =
M->getOrInsertFunction(CmpFnName, B.getInt32Ty(), B.getInt8PtrTy(),
B.getInt8PtrTy(), DL.getIntPtrType(Context));
inferLibFuncAttributes(M, MemCmpName, *TLI);
inferLibFuncAttributes(M, CmpFnName, *TLI);
CallInst *CI = B.CreateCall(
MemCmp, {castToCStr(Ptr1, B), castToCStr(Ptr2, B), Len}, MemCmpName);
CmpFn, {castToCStr(Ptr1, B), castToCStr(Ptr2, B), Len}, CmpFnName);

if (const Function *F =
dyn_cast<Function>(MemCmp.getCallee()->stripPointerCasts()))
dyn_cast<Function>(CmpFn.getCallee()->stripPointerCasts()))
CI->setCallingConv(F->getCallingConv());

return CI;
}

Value *llvm::emitMemCmp(Value *Ptr1, Value *Ptr2, Value *Len, IRBuilder<> &B,
const DataLayout &DL, const TargetLibraryInfo *TLI) {
return emitMemCmpOrBcmp(LibFunc_memcmp, Ptr1, Ptr2, Len, B, DL, TLI);
}

Value *llvm::emitBCmp(Value *Ptr1, Value *Ptr2, Value *Len, IRBuilder<> &B,
const DataLayout &DL, const TargetLibraryInfo *TLI) {
return emitMemCmpOrBcmp(LibFunc_bcmp, Ptr1, Ptr2, Len, B, DL, TLI);
}

/// Append a suffix to the function name according to the type of 'Op'.
static void appendTypeSuffix(Value *Op, StringRef &Name,
SmallString<20> &NameBuffer) {
@@ -831,18 +831,9 @@ Value *LibCallSimplifier::optimizeMemChr(CallInst *CI, IRBuilder<> &B) {
return B.CreateGEP(B.getInt8Ty(), SrcStr, B.getInt64(I), "memchr");
}

Value *LibCallSimplifier::optimizeMemCmp(CallInst *CI, IRBuilder<> &B) {
Value *LHS = CI->getArgOperand(0), *RHS = CI->getArgOperand(1);

if (LHS == RHS) // memcmp(s,s,x) -> 0
return Constant::getNullValue(CI->getType());

// Make sure we have a constant length.
ConstantInt *LenC = dyn_cast<ConstantInt>(CI->getArgOperand(2));
if (!LenC)
return nullptr;

uint64_t Len = LenC->getZExtValue();
static Value *optimizeMemCmpConstantSize(CallInst *CI, Value *LHS, Value *RHS,
uint64_t Len, IRBuilder<> &B,
const DataLayout &DL) {
if (Len == 0) // memcmp(s1,s2,0) -> 0
return Constant::getNullValue(CI->getType());

@@ -912,6 +903,28 @@ Value *LibCallSimplifier::optimizeMemCmp(CallInst *CI, IRBuilder<> &B) {
Ret = 1;
return ConstantInt::get(CI->getType(), Ret);
}
return nullptr;
}

Value *LibCallSimplifier::optimizeMemCmp(CallInst *CI, IRBuilder<> &B) {
Value *LHS = CI->getArgOperand(0), *RHS = CI->getArgOperand(1);
Value *Size = CI->getArgOperand(2);

if (LHS == RHS) // memcmp(s,s,x) -> 0
return Constant::getNullValue(CI->getType());

// Handle constant lengths.
if (ConstantInt *LenC = dyn_cast<ConstantInt>(Size))
if (Value *Res = optimizeMemCmpConstantSize(CI, LHS, RHS,
LenC->getZExtValue(), B, DL))
return Res;

// memcmp(x, y, Len) == 0 -> bcmp(x, y, Len) == 0
// `bcmp` can be more efficient than memcmp because it only has to know that
// there is a difference, not where is is.
if (isOnlyUsedInZeroEqualityComparison(CI) && TLI->has(LibFunc_bcmp)) {
return emitBCmp(LHS, RHS, Size, B, DL, TLI);
}

return nullptr;
}
@@ -1137,10 +1150,10 @@ static Value *optimizeTrigReflections(CallInst *Call, LibFunc Func,
IRBuilder<> &B) {
if (!isa<FPMathOperator>(Call))
return nullptr;

IRBuilder<>::FastMathFlagGuard Guard(B);
B.setFastMathFlags(Call->getFastMathFlags());

// TODO: Can this be shared to also handle LLVM intrinsics?
Value *X;
switch (Func) {
@@ -1340,3 +1340,70 @@ define i32 @huge_length(i8* %X, i8* %Y) nounwind {
%m = tail call i32 @memcmp(i8* %X, i8* %Y, i64 9223372036854775807) nounwind
ret i32 %m
}

define i1 @huge_length_eq(i8* %X, i8* %Y) nounwind {
; X86-LABEL: huge_length_eq:
; X86: # %bb.0:
; X86-NEXT: pushl $2147483647 # imm = 0x7FFFFFFF
; X86-NEXT: pushl $-1
; X86-NEXT: pushl {{[0-9]+}}(%esp)
; X86-NEXT: pushl {{[0-9]+}}(%esp)
; X86-NEXT: calll memcmp
; X86-NEXT: addl $16, %esp
; X86-NEXT: testl %eax, %eax
; X86-NEXT: sete %al
; X86-NEXT: retl
;
; X64-LABEL: huge_length_eq:
; X64: # %bb.0:
; X64-NEXT: pushq %rax
; X64-NEXT: movabsq $9223372036854775807, %rdx # imm = 0x7FFFFFFFFFFFFFFF
; X64-NEXT: callq memcmp
; X64-NEXT: testl %eax, %eax
; X64-NEXT: sete %al
; X64-NEXT: popq %rcx
; X64-NEXT: retq

%m = tail call i32 @memcmp(i8* %X, i8* %Y, i64 9223372036854775807) nounwind
%c = icmp eq i32 %m, 0
ret i1 %c
}

; This checks non-constant sizes.
define i32 @nonconst_length(i8* %X, i8* %Y, i64 %size) nounwind {
; X86-LABEL: nonconst_length:
; X86: # %bb.0:
; X86-NEXT: jmp memcmp # TAILCALL
;
; X64-LABEL: nonconst_length:
; X64: # %bb.0:
; X64-NEXT: jmp memcmp # TAILCALL
%m = tail call i32 @memcmp(i8* %X, i8* %Y, i64 %size) nounwind
ret i32 %m
}

define i1 @nonconst_length_eq(i8* %X, i8* %Y, i64 %size) nounwind {
; X86-LABEL: nonconst_length_eq:
; X86: # %bb.0:
; X86-NEXT: pushl {{[0-9]+}}(%esp)
; X86-NEXT: pushl {{[0-9]+}}(%esp)
; X86-NEXT: pushl {{[0-9]+}}(%esp)
; X86-NEXT: pushl {{[0-9]+}}(%esp)
; X86-NEXT: calll memcmp
; X86-NEXT: addl $16, %esp
; X86-NEXT: testl %eax, %eax
; X86-NEXT: sete %al
; X86-NEXT: retl
;
; X64-LABEL: nonconst_length_eq:
; X64: # %bb.0:
; X64-NEXT: pushq %rax
; X64-NEXT: callq memcmp
; X64-NEXT: testl %eax, %eax
; X64-NEXT: sete %al
; X64-NEXT: popq %rcx
; X64-NEXT: retq
%m = tail call i32 @memcmp(i8* %X, i8* %Y, i64 %size) nounwind
%c = icmp eq i32 %m, 0
ret i1 %c
}
@@ -1,5 +1,5 @@
; RUN: opt < %s -mtriple=x86_64-- -inferattrs -S | FileCheck %s
; RUN: opt < %s -mtriple=x86_64-- -passes=inferattrs -S | FileCheck %s
; RUN: opt < %s -mtriple=x86_64-- -inferattrs -S | FileCheck -check-prefix=CHECK-UNKNOWN %s
; RUN: opt < %s -mtriple=x86_64-- -passes=inferattrs -S | FileCheck -check-prefix=CHECK-UNKNOWN %s
; RUN: opt < %s -mtriple=x86_64-apple-macosx10.8.0 -inferattrs -S | FileCheck -check-prefix=CHECK -check-prefix=CHECK-DARWIN %s
; RUN: opt < %s -mtriple=x86_64-unknown-linux-gnu -inferattrs -S | FileCheck -check-prefix=CHECK -check-prefix=CHECK-LINUX %s
; RUN: opt < %s -mtriple=nvptx -inferattrs -S | FileCheck -check-prefix=CHECK-NVPTX %s
@@ -241,7 +241,10 @@ declare i64 @atol(i8*)
; CHECK: declare i64 @atoll(i8* nocapture) [[G1]]
declare i64 @atoll(i8*)

; CHECK: declare i32 @bcmp(i8* nocapture, i8* nocapture, i64) [[G1]]
; CHECK-DARWIN: declare i32 @bcmp(i8* nocapture, i8* nocapture, i64) [[G1]]
; CHECK-LINUX: declare i32 @bcmp(i8* nocapture, i8* nocapture, i64) [[G1]]
; CHECK-UNKNOWN-NOT: declare i32 @bcmp(i8* nocapture, i8* nocapture, i64) [[G1]]
; CHECK-NVPTX-NOT: declare i32 @bcmp(i8* nocapture, i8* nocapture, i64) [[G1]]
declare i32 @bcmp(i8*, i8*, i64)

; CHECK: declare void @bcopy(i8* nocapture readonly, i8* nocapture, i64) [[G0]]
@@ -1,6 +1,7 @@
; Test that the memcmp library call simplifier works correctly.
;
; RUN: opt < %s -instcombine -S | FileCheck %s
; RUN: opt < %s -instcombine -S | FileCheck --check-prefix=CHECK --check-prefix=NOBCMP %s
; RUN: opt < %s -instcombine -mtriple=x86_64-unknown-linux-gnu -S | FileCheck --check-prefix=CHECK --check-prefix=BCMP %s

target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128-n8:16:32:64"

@@ -130,3 +131,21 @@ define i1 @test_simplify9(i16 %x, i16 %y) {
%cmp = icmp eq i32 %call, 0
ret i1 %cmp
}

; Check memcmp(mem1, mem2, size)==0 -> bcmp(mem1, mem2, size)==0

define i1 @test_simplify10(i8* %mem1, i8* %mem2, i32 %size) {
; NOBCMP-LABEL: @test_simplify10(
; NOBCMP-NEXT: [[CALL:%.*]] = call i32 @memcmp(i8* %mem1, i8* %mem2, i32 %size)
; NOBCMP-NEXT: [[CMP:%.*]] = icmp eq i32 [[CALL]], 0
; NOBCMP-NEXT: ret i1 [[CMP]]
;
; BCMP-LABEL: @test_simplify10(
; BCMP-NEXT: [[CALL:%.*]] = call i32 @bcmp(i8* %mem1, i8* %mem2, i32 %size)
; BCMP-NEXT: [[CMP:%.*]] = icmp eq i32 [[CALL]], 0
; BCMP-NEXT: ret i1 [[CMP]]
;
%call = call i32 @memcmp(i8* %mem1, i8* %mem2, i32 %size)
%cmp = icmp eq i32 %call, 0
ret i1 %cmp
}
@@ -1,5 +1,6 @@
; Test that the strcmp library call simplifier works correctly.
; RUN: opt < %s -instcombine -S | FileCheck %s
; RUN: opt < %s -instcombine -S | FileCheck %s --check-prefix=NOBCMP
; RUN: opt < %s -instcombine -mtriple=unknown-unknown-linux-gnu -S | FileCheck %s --check-prefix=BCMP

target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128"

@@ -80,3 +81,24 @@ define i32 @test6(i8* %str) {
%temp1 = call i32 @strcmp(i8* %str, i8* %str)
ret i32 %temp1
}

; strcmp(x, y) == 0 -> bcmp(x, y, <known length>)
define i1 @test7(i1 %b) {
; BCMP-LABEL: @test7(
; BCMP: %bcmp = call i32 @bcmp(i8* getelementptr inbounds ([6 x i8], [6 x i8]* @hello, i32 0, i32 0), i8* %str2, i32 5)
; BCMP: %res = icmp eq i32 %bcmp, 0
; BCMP: ret i1 %res

; NOBCMP-LABEL: @test7(
; NOBCMP: %memcmp = call i32 @memcmp(i8* getelementptr inbounds ([6 x i8], [6 x i8]* @hello, i32 0, i32 0), i8* %str2, i32 5)
; NOBCMP: %res = icmp eq i32 %memcmp, 0
; NOBCMP: ret i1 %res

%str1 = getelementptr inbounds [6 x i8], [6 x i8]* @hello, i32 0, i32 0
%temp1 = getelementptr inbounds [5 x i8], [5 x i8]* @hell, i32 0, i32 0
%temp2 = getelementptr inbounds [5 x i8], [5 x i8]* @bell, i32 0, i32 0
%str2 = select i1 %b, i8* %temp1, i8* %temp2
%temp3 = call i32 @strcmp(i8* %str1, i8* %str2)
%res = icmp eq i32 %temp3, 0
ret i1 %res
}

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