Re: LRA produces RTL not meeting constraint

[Segher Boessenkool <segher@kernel.crashing.org>]

Hi Paul,

On Tue, Jan 10, 2023 at 02:39:34PM -0500, Paul Koning via Gcc wrote:
> In pdp11.md I have:
> 
> (define_insn_and_split "addhi3"
>   [(set (match_operand:HI 0 "nonimmediate_operand" "=rR,rR,Q,Q")
> 	(plus:HI (match_operand:HI 1 "general_operand" "%0,0,0,0")
> 		 (match_operand:HI 2 "general_operand" "rRLM,Qi,rRLM,Qi")))]
>   ""
>   "#"
>   "reload_completed"
>   [(parallel [(set (match_dup 0)
> 		   (plus:HI (match_dup 1) (match_dup 2)))
> 	      (clobber (reg:CC CC_REGNUM))])]
>   ""
>   [(set_attr "length" "2,4,4,6")])
> 
> While compiling libgcc2.c I see this RTL in the .ira dump file:
> 
> (insn 49 48 53 5 (set (reg/f:HI 136)
>         (plus:HI (reg/f:HI 5 r5)
>             (const_int -8 [0xfffffffffffffff8]))) "../../../../../gcc/libgcc/libgcc2.c":276:4 68 {addhi3}
>      (expr_list:REG_EQUIV (plus:HI (reg/f:HI 5 r5)
>             (const_int -8 [0xfffffffffffffff8]))
>         (nil)))

What hard register was assigned by IRA to r136?  It shows this in the
.ira dump file, search for "Disposition:".

> Then in the .reload dump it appears this way:
> 
> (insn 49 48 53 5 (set (reg/f:HI 5 r5 [136])
>         (plus:HI (reg/f:HI 6 sp)
>             (const_int 40 [0x28]))) "../../../../../gcc/libgcc/libgcc2.c":276:4 68 {addhi3}
>      (expr_list:REG_EQUIV (plus:HI (reg/f:HI 5 r5)
>             (const_int -8 [0xfffffffffffffff8]))
>         (nil)))
> 
> which obviously causes an ICE because that RTL doesn't meet the constraints.

Before reload it did not have operands[0] and operands[1] the same,
already?

> This happens only when LRA is used.
> 
> I also see this in the .reload file, but I don't know what it means:
> 
> 	 Choosing alt 1 in insn 49:  (0) rR  (1) 0  (2) Qi {addhi3}

It chose alternative 1 in this instruction, which uses constraints rR
for operands[0], a tie to that for operands[1], and Qi for operands[2].

>             1 Non-pseudo reload: reject+=2
>             1 Non input pseudo reload: reject++
>             Cycle danger: overall += LRA_MAX_REJECT
>           alt=0,overall=609,losers=1,rld_nregs=2
>             alt=1: Bad operand -- refuse
>             alt=2: Bad operand -- refuse
>           alt=3,overall=0,losers=0,rld_nregs=0

This is for the *next* instruction.  There is a "Choosing alt" thing
for that right after this.  That will be alt 3: 1 and 2 are refused,
0 costs 609, 3 costs 0.

Reading the LRA dumps needs some getting used to ;-)

> Any ideas?  I ran into this when trying to make LRA the default for this target.

Please show *all* relevant things in the reload dump file?  Everything
about insn 49 or any insn added to reload it, for example.


Segher