[PATCH][instcombine] Remove preservesCFG from instcombine

[Nick Lewycky]

Mark Heffernan wrote:
> Hi Andrew,
>
> (Let me know if someone else is more appropriate to review this).  This
> patch removes setPreservesCFG() from the instcombine pass.

NAK. Please don't commit this.

   The reason
> is that InstCombine can modify terminator instructions (branches)
> specifically the predicate.

This is the thing where it removes an '%newcond = xor %cond, true' and 
switches 'br %newcond, label %A, label %B' with 'br %cond, label %B, 
label %A'?

If it's actually causing you problems, please just move that transform 
to simplifycfg instead of removing preserves-cfg from instcombine.

Nick

   Instcombine clears out dead blocks and
> replaces all the dead instruction uses with undef:
>
> http://llvm.org/docs/doxygen/html/InstructionCombining_8cpp_source.html#l02792
>
> These new undefs can be predicates of branches, and from the comment
> of setPreservesCFG():
>
> // setPreservesCFG - This function should be called to by the pass, iff
> they do
> // not:
> //
> //  1. Add or remove basic blocks from the function
> //  2. Modify terminator instructions in any way.
>
> Clearly instcombine is modifying terminator instructions so should not
> be calling setPreservesCFG().  The specific issue I ran into was a dead
> loop not being deleted because of stale analysis data.  Repro:
>
> void foo(int *a, int start_x, int end_x, int start_y, int end_y, int k) {
>    for (int y = start_y; y < end_y; y++) {
>      for (int x = start_x; x < end_x; x++) {
>        for (int i = 0; i < 1000; i++) {
>          a[x + y + k*i] = 0;
>        }
>      }
>    }
> }
>
> # Dead loop left after loop deletion:
> clang -S -emit-llvm foo.c -o - | opt -O1 -loop-rotate -loop-unswitch
> -instcombine -loop-deletion -S > bad.ll
>
> # Dead loop removed by splitting out loop-deletion into separate opt
> invocation:
> clang -S -emit-llvm foo.c -o - | opt -O1 -loop-rotate -loop-unswitch
> -instcombine | opt -loop-deletion -S > good.ll
>
> This patch fixes this discrepancy.
>
> Mark
>
>
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits