[PATCH] D93015: Fold comparison of __builtin_object_size expression with -1 for non-const size

[Siddhesh Poyarekar via Phabricator via llvm-commits]

siddhesh marked an inline comment as done.
siddhesh added inline comments.


================
Comment at: llvm/lib/Analysis/MemoryBuiltins.cpp:549
+      continue;
+
+    ICmpInst::Predicate Pred = CI->getUnsignedPredicate();
----------------
serge-sans-paille wrote:
> This looks very similar to generating a call to `llvm.assume`, right?
> 
> You could generate a call to
> 
> ```
>   %cmpnot = icmp ne i64 %objsize, -1
>   call void @llvm.assume(i1 %cmpnot)
> ```
> 
> And let the magic happen ;-)
I tried, but left it as is for now.  It works fine when it's just `Builder.CreateAssumption(Cond)`, i.e. when the condition is true. However for the false condition, doing `Builder.CreateAssumption(Builder.CreateNot(Cond))` does not work because llvm is unable to work out the relationships and fold the conditions away.  I tried ordering the `assume`, `cmp` and the `cmp.not` instructions in different ways, but no dice.

ISTM the other alternative would be to invert the condition and then update the user branch instructions, thus allowing `Builder.CreateAssumption(Cond)` but that doesn't seem too different from `replaceAllusesWith()`.  What do you think?


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D93015/new/

https://reviews.llvm.org/D93015