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Fix ConstantFold's folding of icmp instructions to recognize that,

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for example, a one-past-the-end pointer from one global variable may
be equal to the base pointer of another global variable.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173995 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman 2013-01-31 00:01:45 +00:00
parent 69f60e7c4a
commit d363ae5299
2 changed files with 28 additions and 6 deletions
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11
lib/IR/ConstantFold.cpp
View File

@ -1495,9 +1495,8 @@ static ICmpInst::Predicate evaluateICmpRelation(Constant *V1, Constant *V2,
"Surprising getelementptr!");
return isSigned ? ICmpInst::ICMP_SGT : ICmpInst::ICMP_UGT;
} else {
// If they are different globals, we don't know what the value is,
// but they can't be equal.
return ICmpInst::ICMP_NE;
// If they are different globals, we don't know what the value is.
return ICmpInst::BAD_ICMP_PREDICATE;
}
}
} else {
@ -1510,10 +1509,10 @@ static ICmpInst::Predicate evaluateICmpRelation(Constant *V1, Constant *V2,
default: break;
case Instruction::GetElementPtr:
// By far the most common case to handle is when the base pointers are
// obviously to the same or different globals.
// obviously to the same global.
if (isa<GlobalValue>(CE1Op0) && isa<GlobalValue>(CE2Op0)) {
if (CE1Op0 != CE2Op0) // Don't know relative ordering, but not equal
return ICmpInst::ICMP_NE;
if (CE1Op0 != CE2Op0) // Don't know relative ordering.
return ICmpInst::BAD_ICMP_PREDICATE;
// Ok, we know that both getelementptr instructions are based on the
// same global. From this, we can precisely determine the relative
// ordering of the resultant pointers.

23
test/Assembler/ConstantExprNoFold.ll Normal file
View File

@ -0,0 +1,23 @@
; This test checks to make sure that constant exprs don't fold in some simple
; situations
; RUN: llvm-as < %s | llvm-dis | FileCheck %s
; Even give it a datalayout, to tempt folding as much as possible.
target datalayout = "p:32:32"
@A = global i64 0
@B = global i64 0
; Don't fold this. @A might really be allocated next to @B, in which case the
; icmp should return true. It's not valid to *dereference* in @B from a pointer
; based on @A, but icmp isn't a dereference.
; CHECK: @C = global i1 icmp eq (i64* getelementptr inbounds (i64* @A, i64 1), i64* @B)
@C = global i1 icmp eq (i64* getelementptr inbounds (i64* @A, i64 1), i64* @B)
; Don't fold this completely away either. In theory this could be simplified
; to only use a gep on one side of the icmp though.
; CHECK: @D = global i1 icmp eq (i64* getelementptr inbounds (i64* @A, i64 1), i64* getelementptr inbounds (i64* @B, i64 2))
@D = global i1 icmp eq (i64* getelementptr inbounds (i64* @A, i64 1), i64* getelementptr inbounds (i64* @B, i64 2))