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If we are checking to see if the result of a call aliases a

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pointer derived from a local allocation, if the local allocation
never escapes, the pointers can't alias.  This implements PR2436


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52301 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2008-06-16 06:19:11 +00:00
parent 8892b6f307
commit 845f0d2f0f
2 changed files with 44 additions and 1 deletions
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22
lib/Analysis/BasicAliasAnalysis.cpp
View File

@ -317,6 +317,18 @@ static bool isKnownNonNull(const Value *V) {
return false;
}
/// isNonEscapingLocalObject - Return true if the pointer is to a function-local
/// object that never escapes from the function.
static bool isNonEscapingLocalObject(const Value *V) {
// If this is a local allocation or byval argument, check to see if it
// escapes.
if (isa<AllocationInst>(V) ||
(isa<Argument>(V) && cast<Argument>(V)->hasByValAttr()))
return !AddressMightEscape(V);
return false;
}
/// isObjectSmallerThan - Return true if we can prove that the object specified
/// by V is smaller than Size.
static bool isObjectSmallerThan(const Value *V, unsigned Size,
@ -393,7 +405,15 @@ BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
(V2Size != ~0U && isObjectSmallerThan(O1, V2Size, TD)))
return NoAlias;
// If one pointer is the result of a call/invoke and the other is a
// non-escaping local object, then we know the object couldn't escape to a
// point where the call could return it.
if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) &&
isNonEscapingLocalObject(O2))
return NoAlias;
if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) &&
isNonEscapingLocalObject(O1))
return NoAlias;
// If we have two gep instructions with must-alias'ing base pointers, figure
// out if the indexes to the GEP tell us anything about the derived pointer.

23
test/Analysis/BasicAA/no-escape-call.ll Normal file
View File

@ -0,0 +1,23 @@
; RUN: llvm-as < %s | opt -basicaa -gvn -instcombine | llvm-dis | grep {ret i1 true}
; PR2436
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"
target triple = "i386-apple-darwin8"
define i1 @foo(i32 %i) nounwind {
entry:
%arr = alloca [10 x i8*] ; <[10 x i8*]*> [#uses=1]
%tmp2 = call i8* @getPtr( ) nounwind ; <i8*> [#uses=2]
%tmp4 = getelementptr [10 x i8*]* %arr, i32 0, i32 %i ; <i8**> [#uses=2]
store i8* %tmp2, i8** %tmp4, align 4
%tmp10 = getelementptr i8* %tmp2, i32 10 ; <i8*> [#uses=1]
store i8 42, i8* %tmp10, align 1
%tmp14 = load i8** %tmp4, align 4 ; <i8*> [#uses=1]
%tmp16 = getelementptr i8* %tmp14, i32 10 ; <i8*> [#uses=1]
%tmp17 = load i8* %tmp16, align 1 ; <i8> [#uses=1]
%tmp19 = icmp eq i8 %tmp17, 42 ; <i1> [#uses=1]
ret i1 %tmp19
}
declare i8* @getPtr()
declare void @abort() noreturn nounwind