[PATCH] D13974: [IndVarSimplify] Rewrite loop exit values with their initial values from loop preheader

[Chen Li via llvm-commits]

chenli added inline comments.

================
Comment at: lib/Transforms/Scalar/IndVarSimplify.cpp:747
@@ +746,3 @@
+
+        // All non-instructions are loop-invariant.
+        if (auto *CondInst = dyn_cast<Instruction>(Cond))
----------------
sanjoy wrote:
> ]I'd change this to `isLoopInvariant` for now -- `make LoopInvariant` may hoist arithmetic etc. over control flow and this could be a regression e.g. in cases like
> 
> 
> ```
> for ( ... )
>   if (rare_condition) {
>     int x = loop_invariant * 3;
>     if (x == 42) then exit;
>   }
> ```
> 
> hoisting `loop_invariant * 3` to the loop preheader will be a net regression.
> 
> Sorry for not bringing this up earlier.
> 
> Also, note that both `isLoopInvariant` and `makeLoopInvariant` take a `Value` so you don't need to manually cast to an `Instruction`.
By regression, do you mean by register pressure?


http://reviews.llvm.org/D13974