[llvm] r219688 - [LVI] Check for @llvm.assume dominating the edge branch

[Philip Reames]

Hal,

We had a non-trivial conversation about why using the CxtI information 
was valid at all.  Would you mind adding a comment to that effect?  
Otherwise, I'm going to think there's a bug here every time I glance at 
this code.  :)

Philip

On 10/14/2014 09:04 AM, Hal Finkel wrote:
> Author: hfinkel
> Date: Tue Oct 14 11:04:49 2014
> New Revision: 219688
>
> URL: http://llvm.org/viewvc/llvm-project?rev=219688&view=rev
> Log:
> [LVI] Check for @llvm.assume dominating the edge branch
>
> 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!
>
> Added:
>      llvm/trunk/test/Transforms/JumpThreading/assume-edge-dom.ll
> Modified:
>      llvm/trunk/lib/Analysis/LazyValueInfo.cpp
>
> Modified: llvm/trunk/lib/Analysis/LazyValueInfo.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/LazyValueInfo.cpp?rev=219688&r1=219687&r2=219688&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Analysis/LazyValueInfo.cpp (original)
> +++ llvm/trunk/lib/Analysis/LazyValueInfo.cpp Tue Oct 14 11:04:49 2014
> @@ -954,6 +954,7 @@ bool LazyValueInfoCache::getEdgeValue(Va
>   
>       // 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(Va
>   
>     // 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;
>   }
>
> Added: llvm/trunk/test/Transforms/JumpThreading/assume-edge-dom.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/JumpThreading/assume-edge-dom.ll?rev=219688&view=auto
> ==============================================================================
> --- llvm/trunk/test/Transforms/JumpThreading/assume-edge-dom.ll (added)
> +++ llvm/trunk/test/Transforms/JumpThreading/assume-edge-dom.ll Tue Oct 14 11:04:49 2014
> @@ -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
> +}
> +
>
>
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