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[LVI] Check for @llvm.assume dominating the edge branch

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When LazyValueInfo uses @llvm.assume intrinsics to provide edge-value
constraints, we should check for intrinsics that dominate the edge's branch,
not just any potential context instructions. An assumption that dominates the
edge's branch represents a truth on that edge. This is specifically useful, for
example, if multiple predecessors assume a pointer to be nonnull, allowing us
to simplify a later null comparison.

The test case, and an initial patch, were provided by Philip Reames. Thanks!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219688 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Hal Finkel 2014-10-14 16:04:49 +00:00
parent 65e4aa4656
commit 2993617e41
2 changed files with 41 additions and 0 deletions
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2
lib/Analysis/LazyValueInfo.cpp
View File

@ -954,6 +954,7 @@ bool LazyValueInfoCache::getEdgeValue(Value *Val, BasicBlock *BBFrom,
// Try to intersect ranges of the BB and the constraint on the edge.
LVILatticeVal InBlock = getBlockValue(Val, BBFrom);
mergeAssumeBlockValueConstantRange(Val, InBlock, BBFrom->getTerminator());
mergeAssumeBlockValueConstantRange(Val, InBlock, CxtI);
if (!InBlock.isConstantRange())
return true;
@ -971,6 +972,7 @@ bool LazyValueInfoCache::getEdgeValue(Value *Val, BasicBlock *BBFrom,
// if we couldn't compute the value on the edge, use the value from the BB
Result = getBlockValue(Val, BBFrom);
mergeAssumeBlockValueConstantRange(Val, Result, BBFrom->getTerminator());
mergeAssumeBlockValueConstantRange(Val, Result, CxtI);
return true;
}

39
test/Transforms/JumpThreading/assume-edge-dom.ll Normal file
View File

@ -0,0 +1,39 @@
; RUN: opt -S -jump-threading < %s | FileCheck %s
declare i8* @escape()
declare void @llvm.assume(i1)
define i1 @test1(i1 %cond) {
entry:
br i1 %cond, label %taken, label %not_taken
; CHECK-LABEL: @test1
; CHECK: br i1 %cond, label %no, label %yes
; CHECK: ret i1 true
taken:
%res1 = call i8* @escape()
%a = icmp eq i8* %res1, null
tail call void @llvm.assume(i1 %a)
br label %done
not_taken:
%res2 = call i8* @escape()
%b = icmp ne i8* %res2, null
tail call void @llvm.assume(i1 %b)
br label %done
; An assume that can be used to simplify this comparison dominates each
; predecessor branch (although no assume dominates the cmp itself). Make sure
; this still can be simplified.
done:
%res = phi i8* [ %res1, %taken ], [ %res2, %not_taken ]
%cnd = icmp ne i8* %res, null
br i1 %cnd, label %yes, label %no
yes:
ret i1 true
no:
ret i1 false
}