[llvm] r215943 - Answer to Philip Reames comments
Robin Morisset
morisset at google.com
Mon Aug 18 15:18:14 PDT 2014
Author: morisset
Date: Mon Aug 18 17:18:14 2014
New Revision: 215943
URL: http://llvm.org/viewvc/llvm-project?rev=215943&view=rev
Log:
Answer to Philip Reames comments
- 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
Modified:
llvm/trunk/lib/Analysis/MemoryDependenceAnalysis.cpp
llvm/trunk/test/Transforms/GVN/atomic.ll
Modified: llvm/trunk/lib/Analysis/MemoryDependenceAnalysis.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/MemoryDependenceAnalysis.cpp?rev=215943&r1=215942&r2=215943&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/MemoryDependenceAnalysis.cpp (original)
+++ llvm/trunk/lib/Analysis/MemoryDependenceAnalysis.cpp Mon Aug 18 17:18:14 2014
@@ -407,21 +407,33 @@ getPointerDependencyFrom(const AliasAnal
// 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 AliasAnal
// 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.
Modified: llvm/trunk/test/Transforms/GVN/atomic.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/GVN/atomic.ll?rev=215943&r1=215942&r2=215943&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/GVN/atomic.ll (original)
+++ llvm/trunk/test/Transforms/GVN/atomic.ll Mon Aug 18 17:18:14 2014
@@ -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
+}
+
+
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