[llvm] r336172 - [InstCombine] Delay foldICmpUsingKnownBits until simple transforms are done

[Friedman, Eli via llvm-commits]

On 7/2/2018 11:23 PM, Max Kazantsev via llvm-commits wrote:
> Author: mkazantsev
> Date: Mon Jul  2 23:23:57 2018
> New Revision: 336172
>
> URL: http://llvm.org/viewvc/llvm-project?rev=336172&view=rev
> Log:
> [InstCombine] Delay foldICmpUsingKnownBits until simple transforms are done
>
> This patch changes order of transform in InstCombineCompares to avoid
> performing transforms based on ranges which produce complex bit arithmetics
> before more simple things (like folding with constants) are done. See PR37636
> for the motivating example.
>
> Differential Revision: https://reviews.llvm.org/D48584
> Reviewed By: spatel, lebedev.ri

This patch seems to be leading to worse generated code in certain cases 
because it prefers ugt over eq in cases where they're interchangeable. 
I'm seeing diffs like the following:

         %40 = lshr i8 %39, 1
     >   %cmp8.i = icmp ugt i8 %39, 1, !dbg !53520
     >   %narrow.i = select i1 %cmp8.i, i8 %40, i8 1, !dbg !53521
     <   %cmp8.i = icmp eq i8 %40, 0, !dbg !53520
     <   %narrow.i = select i1 %cmp8.i, i8 1, i8 %40, !dbg !53521

On Thumb1, leads to an extra "cmp", instead of using flags produced by lsrs.

     >   %bf.cast1182.i = and i88 %bf.load115.i, 71776119061217280, !dbg 
!53502
     >   %cmp119.i = icmp ugt i88 %bf.cast1182.i, 281474976710656, !dbg 
!53502
     <   %bf.cast1182.i = and i88 %bf.load115.i, 71494644084506624, !dbg 
!53502
     <   %cmp119.i = icmp eq i88 %bf.cast1182.i, 0, !dbg !53502
         %39 = load i64, i64* %11, align 8, !dbg !53503
     >   br i1 %cmp119.i, label %if.then121.i, label %if.end132.i, !dbg 
!53504
     <   br i1 %cmp119.i, label %if.end132.i, label %if.then121.i, !dbg 
!53504

On Thumb1, leads to materializing multiple expensive immediates, instead 
of a couple simple shifts.

Alive proof the two forms are equivalent: https://rise4fun.com/Alive/cEq

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project