[llvm-dev] Tail merging "undef" with a defined register: wrong code

[Krzysztof Parzyszek via llvm-dev]

On 7/28/2017 1:59 PM, Quentin Colombet wrote:
> Hi Krzysztof,
> 
> Thanks for digging into this.
> 
>> On Jul 28, 2017, at 11:08 AM, Krzysztof Parzyszek <kparzysz at codeaurora.org> wrote:
>>
>> I've looked into that and it's not going to be simple, unfortunately.
>>
>> Here's the original example again:
>>
>> ---
>> 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
>>       %r0 = L2_loadruh_io undef %r0, 0
>>       PS_storerhabs 0, killed %r0
>>       J2_jump %bb.3, implicit-def %pc
>>
>>   bb.2:
>>     successors: %bb.3
>>       PS_storerhabs 0, undef %r0
>>       J2_jump %bb.3, implicit-def %pc
>>
>>   bb.3:
>>       PS_jmpret killed %r31, implicit-def %pc
>> ...
>>
>>
>> If the merging process produced this intermediate code (I stripped some unimportant pieces):
>>
>>   bb.1:
>>       %r0 = L2_loadruh_io undef %r0, 0
>>       J2_jump %bb.tail
>>
>>   bb.2:
>>       J2_jump %bb.tail
>>
>>   bb.tail:
>>     liveins: %r0
>>       PS_storerhabs 0, killed %r0
>>       J2_jump %bb.3, implicit-def %pc
>>
>> Then we could insert IMPLICIT_DEF into bb.2 and everything would be fine. This is not what happens, though. In this case, the entire bb.2 will be used as the new tail leaving us without a good point to put the implicit def at.
> 
> We should need to put the implicit_def only if the related register is dead (as in not live-out).

Yes.

> Thus, for this case we wouldn’t need to do anything, right?

Why not? This is the same testcase as in the beginning of the thread. 
In the actual output produced by tail merging, r0 is not live out of 
bb.0, but is live on entry to one of its successors. Even with the 
hypothetical code, r0 is live on entry to bb.tail, but not live on exit 
from bb.2. Both cases need some fixes.

Since I've removed the old quotes for brevity, this is the current output:

body: |
   bb.0:
     successors: %bb.1, %bb.2
       J2_jumpt undef %p0, %bb.2, implicit-def %pc

   bb.1:
     successors: %bb.2
       %r0 = L2_loadruh_io undef %r0, 0

   bb.2:
     liveins: %r0
       PS_storerhabs 0, %r0
       PS_jmpret killed %r31, implicit-def %pc
...


-Krzysztof


-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, 
hosted by The Linux Foundation