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Answer to Philip Reames comments

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- add check for volatile (probably unneeded, but I agree that we should be conservative about it).
- strengthen condition from isUnordered() to isSimple(), as I don't understand well enough Unordered semantics (and it also matches the comment better this way) to be confident in the previous behaviour (thanks for catching that one, I had missed the case Monotonic/Unordered).
- separate a condition in two.
- lengthen comment about aliasing and loads
- add tests in GVN/atomic.ll

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215943 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Robin Morisset 2014-08-18 22:18:14 +00:00
parent 6c0e1e0fa6
commit 0acd42142a
2 changed files with 53 additions and 6 deletions
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33
lib/Analysis/MemoryDependenceAnalysis.cpp
View File

@ -407,21 +407,33 @@ getPointerDependencyFrom(const AliasAnalysis::Location &MemLoc, bool isLoad,
// Values depend on loads if the pointers are must aliased. This means that
// a load depends on another must aliased load from the same value.
// One exception is atomic loads: a value can depend on an atomic load that it
// does not alias with when this atomic load indicates that another thread may
// be accessing the location.
if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
// Atomic loads have complications involved.
// A monotonic load is OK if the query inst is itself not atomic.
// FIXME: This is overly conservative.
if (!LI->isUnordered()) {
if (!QueryInst || LI->getOrdering() != Monotonic)
if (!QueryInst)
return MemDepResult::getClobber(LI);
if (LI->getOrdering() != Monotonic)
return MemDepResult::getClobber(LI);
if (auto *QueryLI = dyn_cast<LoadInst>(QueryInst))
if (!QueryLI->isUnordered())
if (!QueryLI->isSimple())
return MemDepResult::getClobber(LI);
if (auto *QuerySI = dyn_cast<StoreInst>(QueryInst))
if (!QuerySI->isUnordered())
if (!QuerySI->isSimple())
return MemDepResult::getClobber(LI);
}
// FIXME: this is overly conservative.
// While volatile access cannot be eliminated, they do not have to clobber
// non-aliasing locations, as normal accesses can for example be reordered
// with volatile accesses.
if (LI->isVolatile())
return MemDepResult::getClobber(LI);
AliasAnalysis::Location LoadLoc = AA->getLocation(LI);
// If we found a pointer, check if it could be the same as our pointer.
@ -481,16 +493,25 @@ getPointerDependencyFrom(const AliasAnalysis::Location &MemLoc, bool isLoad,
// A monotonic store is OK if the query inst is itself not atomic.
// FIXME: This is overly conservative.
if (!SI->isUnordered()) {
if (!QueryInst || SI->getOrdering() != Monotonic)
if (!QueryInst)
return MemDepResult::getClobber(SI);
if (SI->getOrdering() != Monotonic)
return MemDepResult::getClobber(SI);
if (auto *QueryLI = dyn_cast<LoadInst>(QueryInst))
if (!QueryLI->isUnordered())
if (!QueryLI->isSimple())
return MemDepResult::getClobber(SI);
if (auto *QuerySI = dyn_cast<StoreInst>(QueryInst))
if (!QuerySI->isUnordered())
if (!QuerySI->isSimple())
return MemDepResult::getClobber(SI);
}
// FIXME: this is overly conservative.
// While volatile access cannot be eliminated, they do not have to clobber
// non-aliasing locations, as normal accesses can for example be reordered
// with volatile accesses.
if (SI->isVolatile())
return MemDepResult::getClobber(SI);
// If alias analysis can tell that this store is guaranteed to not modify
// the query pointer, ignore it. Use getModRefInfo to handle cases where
// the query pointer points to constant memory etc.

26
test/Transforms/GVN/atomic.ll
View File

@ -78,3 +78,29 @@ entry:
%x3 = add i32 %x, %x2
ret i32 %x3
}
; GVN across monotonic store (allowed)
define i32 @test7() nounwind uwtable ssp {
; CHECK: test7
; CHECK: add i32 %x, %x
entry:
%x = load i32* @y
store atomic i32 %x, i32* @x monotonic, align 4
%y = load i32* @y
%z = add i32 %x, %y
ret i32 %z
}
; GVN of an unordered across monotonic load (not allowed)
define i32 @test8() nounwind uwtable ssp {
; CHECK: test8
; CHECK: add i32 %x, %y
entry:
%x = load atomic i32* @y unordered, align 4
%clobber = load atomic i32* @x monotonic, align 4
%y = load atomic i32* @y monotonic, align 4
%z = add i32 %x, %y
ret i32 %z
}