[llvm-dev] TryToShrinkGlobalToBoolean in GlobalOpt.cpp issue

[Chris Lattner via llvm-dev]

On Aug 30, 2016, at 1:01 PM, Ryan Taylor via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Given some code:
> 
> static int x = 100;
> 
> int main() {
>    x = 101;
>    printf("%d\n", x);
> }
> 
> results in:
> 
> %0 = load i1
> %1 = select %0, 101, 100
> ...
> ...
> 
> 1) What architecture(s) does this benefit?

Probably none of them.

The intent of this (very very old) optimization is to expose more value information to the mid-level optimizer, which enables secondary simplification.  If this simplification doesn’t happen, then this is probably almost always a loss.

A better way to handle this is for GlobalOpt.cpp to tag the loads with the “new” range metadata: http://llvm.org/docs/LangRef.html#range-metadata

In addition to this not pessimizing code, this would allow the analysis to be more general: instead of identifying 1 or 2 constants, it could be generalized to N ranges.

-Chris