[PATCH] D16240: [InstCombine] Simplify a known nonzero incoming value of PHI

[Jun Bum Lim via llvm-commits]

junbuml added inline comments.

================
Comment at: lib/Transforms/InstCombine/InstCombinePHI.cpp:940
@@ +939,3 @@
+        Value *VA = PN.getIncomingValue(i);
+        if (!isa<ConstantInt>(VA) && VA->hasOneUse() &&
+            isKnownNonZero(VA, DL, 0, AC, &PN, DT))
----------------
mssimpso wrote:
> junbuml wrote:
> > sanjoy wrote:
> > > Why do you care about `VA` having only one use?
> > In correctness perspective, we don't have to check VA->hasOneUse(), but I expected the real impact  on performance only when the incoming value become a dead code from this modification. I'm okay with removing this check (VA->hasOneUse()) if we can expect any indirect benefit.
> I think keeping the one-use check makes sense. If we only expect there to be a benefit if VA becomes dead, we want to avoid increasing compile-time. The only corner case that I can think of is if all of VAs users are PHIs that are compared with zero.
> The only corner case that I can think of is if all of VAs users are PHIs that are compared with zero.

This could a possible corner case, but to be simple for now, I may want to handle it as an extension if we need it.


http://reviews.llvm.org/D16240