[llvm] r266200 - Simplify strlen to a subtraction for certain cases.
David L Kreitzer via llvm-commits
llvm-commits at lists.llvm.org
Wed Apr 13 07:31:06 PDT 2016
Author: dlkreitz
Date: Wed Apr 13 09:31:06 2016
New Revision: 266200
URL: http://llvm.org/viewvc/llvm-project?rev=266200&view=rev
Log:
Simplify strlen to a subtraction for certain cases.
Patch by Li Huang (li1.huang at intel.com)
Differential Revision: http://reviews.llvm.org/D18230
Modified:
llvm/trunk/include/llvm/Analysis/ValueTracking.h
llvm/trunk/lib/Analysis/ValueTracking.cpp
llvm/trunk/lib/Transforms/Utils/SimplifyLibCalls.cpp
llvm/trunk/test/Transforms/InstCombine/strlen-1.ll
Modified: llvm/trunk/include/llvm/Analysis/ValueTracking.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/ValueTracking.h?rev=266200&r1=266199&r2=266200&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Analysis/ValueTracking.h (original)
+++ llvm/trunk/include/llvm/Analysis/ValueTracking.h Wed Apr 13 09:31:06 2016
@@ -26,6 +26,7 @@ namespace llvm {
class AssumptionCache;
class DataLayout;
class DominatorTree;
+ class GEPOperator;
class Instruction;
class Loop;
class LoopInfo;
@@ -183,6 +184,10 @@ namespace llvm {
DL);
}
+ /// Returns true if the GEP is based on a pointer to a string (array of i8),
+ /// and is indexing into this string.
+ bool isGEPBasedOnPointerToString(const GEPOperator *GEP);
+
/// getConstantStringInfo - This function computes the length of a
/// null-terminated C string pointed to by V. If successful, it returns true
/// and returns the string in Str. If unsuccessful, it returns false. This
Modified: llvm/trunk/lib/Analysis/ValueTracking.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ValueTracking.cpp?rev=266200&r1=266199&r2=266200&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ValueTracking.cpp (original)
+++ llvm/trunk/lib/Analysis/ValueTracking.cpp Wed Apr 13 09:31:06 2016
@@ -2650,6 +2650,24 @@ Value *llvm::GetPointerBaseWithConstantO
return Ptr;
}
+bool llvm::isGEPBasedOnPointerToString(const GEPOperator *GEP) {
+ // Make sure the GEP has exactly three arguments.
+ if (GEP->getNumOperands() != 3)
+ return false;
+
+ // Make sure the index-ee is a pointer to array of i8.
+ ArrayType *AT = dyn_cast<ArrayType>(GEP->getSourceElementType());
+ if (!AT || !AT->getElementType()->isIntegerTy(8))
+ return false;
+
+ // Check to make sure that the first operand of the GEP is an integer and
+ // has value 0 so that we are sure we're indexing into the initializer.
+ const ConstantInt *FirstIdx = dyn_cast<ConstantInt>(GEP->getOperand(1));
+ if (!FirstIdx || !FirstIdx->isZero())
+ return false;
+
+ return true;
+}
/// This function computes the length of a null-terminated C string pointed to
/// by V. If successful, it returns true and returns the string in Str.
@@ -2664,19 +2682,9 @@ bool llvm::getConstantStringInfo(const V
// If the value is a GEP instruction or constant expression, treat it as an
// offset.
if (const GEPOperator *GEP = dyn_cast<GEPOperator>(V)) {
- // Make sure the GEP has exactly three arguments.
- if (GEP->getNumOperands() != 3)
- return false;
-
- // Make sure the index-ee is a pointer to array of i8.
- ArrayType *AT = dyn_cast<ArrayType>(GEP->getSourceElementType());
- if (!AT || !AT->getElementType()->isIntegerTy(8))
- return false;
-
- // Check to make sure that the first operand of the GEP is an integer and
- // has value 0 so that we are sure we're indexing into the initializer.
- const ConstantInt *FirstIdx = dyn_cast<ConstantInt>(GEP->getOperand(1));
- if (!FirstIdx || !FirstIdx->isZero())
+ // The GEP operator should be based on a pointer to string constant, and is
+ // indexing into the string constant.
+ if (!isGEPBasedOnPointerToString(GEP))
return false;
// If the second index isn't a ConstantInt, then this is a variable index
Modified: llvm/trunk/lib/Transforms/Utils/SimplifyLibCalls.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/SimplifyLibCalls.cpp?rev=266200&r1=266199&r2=266200&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/SimplifyLibCalls.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/SimplifyLibCalls.cpp Wed Apr 13 09:31:06 2016
@@ -535,6 +535,57 @@ Value *LibCallSimplifier::optimizeStrLen
if (uint64_t Len = GetStringLength(Src))
return ConstantInt::get(CI->getType(), Len - 1);
+ // If s is a constant pointer pointing to a string literal, we can fold
+ // strlen(s + x) to strlen(s) - x, when x is known to be in the range
+ // [0, strlen(s)] or the string has a single null terminator '\0' at the end.
+ // We only try to simplify strlen when the pointer s points to an array
+ // of i8. Otherwise, we would need to scale the offset x before doing the
+ // subtraction. This will make the optimization more complex, and it's not
+ // very useful because calling strlen for a pointer of other types is
+ // very uncommon.
+ if (GEPOperator *GEP = dyn_cast<GEPOperator>(Src)) {
+ if (!isGEPBasedOnPointerToString(GEP))
+ return nullptr;
+
+ StringRef Str;
+ if (getConstantStringInfo(GEP->getOperand(0), Str, 0, false)) {
+ size_t NullTermIdx = Str.find('\0');
+
+ // If the string does not have '\0', leave it to strlen to compute
+ // its length.
+ if (NullTermIdx == StringRef::npos)
+ return nullptr;
+
+ Value *Offset = GEP->getOperand(2);
+ unsigned BitWidth = Offset->getType()->getIntegerBitWidth();
+ APInt KnownZero(BitWidth, 0);
+ APInt KnownOne(BitWidth, 0);
+ computeKnownBits(Offset, KnownZero, KnownOne, DL, 0, nullptr, CI,
+ nullptr);
+ KnownZero.flipAllBits();
+ size_t ArrSize =
+ cast<ArrayType>(GEP->getSourceElementType())->getNumElements();
+
+ // KnownZero's bits are flipped, so zeros in KnownZero now represent
+ // bits known to be zeros in Offset, and ones in KnowZero represent
+ // bits unknown in Offset. Therefore, Offset is known to be in range
+ // [0, NullTermIdx] when the flipped KnownZero is non-negative and
+ // unsigned-less-than NullTermIdx.
+ //
+ // If Offset is not provably in the range [0, NullTermIdx], we can still
+ // optimize if we can prove that the program has undefined behavior when
+ // Offset is outside that range. That is the case when GEP->getOperand(0)
+ // is a pointer to an object whose memory extent is NullTermIdx+1.
+ if ((KnownZero.isNonNegative() && KnownZero.ule(NullTermIdx)) ||
+ (GEP->isInBounds() && isa<GlobalVariable>(GEP->getOperand(0)) &&
+ NullTermIdx == ArrSize - 1))
+ return B.CreateSub(ConstantInt::get(CI->getType(), NullTermIdx),
+ Offset);
+ }
+
+ return nullptr;
+ }
+
// strlen(x?"foo":"bars") --> x ? 3 : 4
if (SelectInst *SI = dyn_cast<SelectInst>(Src)) {
uint64_t LenTrue = GetStringLength(SI->getTrueValue());
Modified: llvm/trunk/test/Transforms/InstCombine/strlen-1.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/strlen-1.ll?rev=266200&r1=266199&r2=266200&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/strlen-1.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/strlen-1.ll Wed Apr 13 09:31:06 2016
@@ -1,4 +1,4 @@
-; NOTE: Assertions have been autogenerated by update_test_checks.py
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; Test that the strlen library call simplifier works correctly.
;
; RUN: opt < %s -instcombine -S | FileCheck %s
@@ -113,9 +113,8 @@ define i32 @test_simplify9(i1 %x) {
define i32 @test_simplify10(i32 %x) {
; CHECK-LABEL: @test_simplify10(
-; CHECK-NEXT: [[HELLO_P:%.*]] = getelementptr inbounds [6 x i8], [6 x i8]* @hello, i32 0, i32 %x
-; CHECK-NEXT: [[HELLO_L:%.*]] = call i32 @strlen(i8* [[HELLO_P]])
-; CHECK-NEXT: ret i32 [[HELLO_L]]
+; CHECK-NEXT: [[TMP1:%.*]] = sub i32 5, %x
+; CHECK-NEXT: ret i32 [[TMP1]]
;
%hello_p = getelementptr inbounds [6 x i8], [6 x i8]* @hello, i32 0, i32 %x
%hello_l = call i32 @strlen(i8* %hello_p)
@@ -127,9 +126,8 @@ define i32 @test_simplify10(i32 %x) {
define i32 @test_simplify11(i32 %x) {
; CHECK-LABEL: @test_simplify11(
; CHECK-NEXT: [[AND:%.*]] = and i32 %x, 7
-; CHECK-NEXT: [[HELLO_P:%.*]] = getelementptr inbounds [13 x i8], [13 x i8]* @null_hello_mid, i32 0, i32 [[AND]]
-; CHECK-NEXT: [[HELLO_L:%.*]] = call i32 @strlen(i8* [[HELLO_P]])
-; CHECK-NEXT: ret i32 [[HELLO_L]]
+; CHECK-NEXT: [[TMP1:%.*]] = sub nsw i32 9, [[AND]]
+; CHECK-NEXT: ret i32 [[TMP1]]
;
%and = and i32 %x, 7
%hello_p = getelementptr inbounds [13 x i8], [13 x i8]* @null_hello_mid, i32 0, i32 %and
More information about the llvm-commits
mailing list