[llvm-commits] [llvm] r97126 - in /llvm/trunk: lib/Analysis/ScalarEvolution.cpp lib/Transforms/Scalar/IndVarSimplify.cpp lib/Transforms/Utils/LoopSimplify.cpp test/CodeGen/X86/2009-09-07-CoalescerBug.ll test/Transforms/IndVarSimplify/2003-09-12-MultiplePred.ll test/Transforms/LoopDeletion/simplify-then-delete.ll

[Dan Gohman]

On Feb 24, 2010, at 11:12 PM, Nick Lewycky wrote:

> Dan Gohman wrote:
>> Author: djg
>> Date: Thu Feb 25 00:57:05 2010
>> New Revision: 97126
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=97126&view=rev
>> Log:
>> Make LoopSimplify change conditional branches in loop exiting blocks
>> which branch on undef to branch on a boolean constant for the edge
>> exiting the loop. This helps ScalarEvolution compute trip counts for
>> loops.
>
> I'm confused. 'undef' is already a constant and ought to be more  
> useful for optimization than i1 0 or i1 1. Are you sure this is the  
> right fix?

If ScalarEvolution picks "false" and later JumpThreading, picks "true",
badness could ensue.  By modifying the branch, LoopSimplify
ensures that all subsequent passes will use the same decision.

Dan