[PATCH] D37034: Insert IMPLICIT_DEFS for undef uses in tail merging

[Krzysztof Parzyszek via Phabricator via llvm-commits]

kparzysz added inline comments.


================
Comment at: lib/CodeGen/BranchFolding.cpp:379
+      LiveOuts.clear();
+      LiveOuts.addLiveOuts(*Pred);
 
----------------
kparzysz wrote:
> This needs to calculate liveouts with the tail removed.
> 
> This testcase still fails:
> 
> ```
> # RUN: llc -march=hexagon -run-pass branch-folder %s -o - 
> ---
> name: fred
> tracksRegLiveness: true
> 
> body: |
>   bb.0:
>     successors: %bb.1, %bb.2
>       J2_jumpt undef %p0, %bb.2, implicit-def %pc
>       J2_jump %bb.1, implicit-def %pc
> 
>   bb.1:
>     successors: %bb.3
>       %r1 = A2_tfrsi 1
>       PS_storerhabs 0, undef %r0
>       %r0 = A2_tfrsi 1
>       J2_jump %bb.3, implicit-def %pc
> 
>   bb.2:
>     successors: %bb.3
>       %r0 = L2_loadruh_io undef %r1, 0
>       PS_storerhabs 0, killed %r0
>       %r0 = A2_tfrsi 1
>       J2_jump %bb.3, implicit-def %pc
> 
>   bb.3:
>       PS_jmpret killed %r31, implicit undef %r0, implicit-def %pc
> ...
> ```
> 
> The output is
> 
> ```
> body:             |
>   bb.0:
>     successors: %bb.1(0x40000000), %bb.2(0x40000000)
>   
>     J2_jumpt undef %p0, %bb.2, implicit-def %pc
>   
>   bb.1:
>     successors: %bb.3(0x80000000)
>   
>     %r1 = A2_tfrsi 1
>     J2_jump %bb.3, implicit-def %pc
>   
>   bb.2:
>     successors: %bb.3(0x80000000)
>   
>     %r0 = L2_loadruh_io undef %r1, 0
>   
>   bb.3:
>     liveins: %r0
>   
>     PS_storerhabs 0, killed %r0
>     %r0 = A2_tfrsi 1
>     PS_jmpret killed %r31, implicit undef %r0, implicit-def %pc
> 
> ...
> ```
> 
> The path `bb.0` -> `bb.1` -> `bb.3` does not define `r0`, but `r0` is a live-in to `bb.3`.
In the original testcase, one of the blocks became the common tail.  In this testcase, r1=... was added to that block to force a fresh block to be created for the tail. Then r0=... was added to both source blocks, but at the end so that it would be a part of the common tail.  Because of how the live-outs in the code are calculated, this would prevent implicit defs from being added.

I had to change the order of the blocks to trigger the problem, since the traversal order for the predecessors may avoid exposing it.


Repository:
  rL LLVM

https://reviews.llvm.org/D37034