MIPS - Filling delay slots

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1 
Hi Daniel,


The Clang with LLVM backend seems to be missing some opportunities to fill branch delay slots for the MIPS M14K processor.


Example:
static inline void LOCAL(encode_position)(VARS)
{
  if (vars->is_mcs05) {
    udi(vpu_find_first_neg_from_reduction_4_mcs05, 1, 0);
  } else {
    udi(vpu_find_first_neg_from_reduction_4, 1, 0);
  }
}

is giving

bfc03394:	12800004 	beqz	s4,bfc033a8 <rx4_eostf+0x4a4>
bfc03398:	00000000 	nop
bfc0339c:	70016450 	udi	VPU,vpu_find_first_neg_from_reduction_4_mcs05,1,zero
bfc033a0:	0bf00ceb 	j	bfc033ac <rx4_eostf+0x4a8>
bfc033a4:	00000000 	nop
bfc033a8:	70016350 	udi	VPU,vpu_find_first_neg_from_reduction_4,1,zero
bfc033ac:	24012200 	li	at,8704
bfc033b0:	72201bd0 	udi	VPU,wait_req_pending,1,s1,0,0

I think the following is possible

beqz	s4,1f
li	at,8704 ; **DELAY SLOT**
j	2f
udi	VPU,vpu_find_first_neg_from_reduction_4_mcs05,1,zero ; **DELAY SLOT**
1:
udi	VPU,vpu_find_first_neg_from_reduction_4,1,zero
2:
udi	VPU,wait_req_pending,1,s1,0,0



Is there something I could try to exploit delay slots in branch instructions or are there any recent patch which deals with this?


Thanks,


Ambuj
2

beqz s4,1f

li at,8704 ; **DELAY SLOT**

j 2f

udi VPU,vpu_find_first_neg_from_reduction_4_mcs05,1,zero ; **DELAY SLOT**

1:

udi VPU,vpu_find_first_neg_from_reduction_4,1,zero

2:

udi VPU,wait_req_pending,1,s1,0,0

Is there something I could try to exploit delay slots in branch instructions or are there any recent patch which deals with this?

It looks like you're pulling instructions from the successor blocks into the delay slot. This is valid (with some exceptions like branches) and there is code in our backend to do it. However, it's not enabled by default at the moment because it has some problems.
The feature is guarded by -disable-mips-df-forward-search and -disable-mips-df-succbb-search (which both default to true) if you want to try it.

Also relevant: If you're on the trunk then you'll already have this Vasileios noticed an issue recently with KILL instructions preventing opportunities to fill the delay slot. This was fixed in r235183 (rGbb60cfb5c4c9).