Re: [ARM] Fix build failure due to movsi_compare0 (PR 61430)

[Chung-Lin Tang <cltang at codesourcery dot com>]

On 2014/6/11 äå 06:32, James Greenhalgh wrote:
> 
> Hi,
> 
> A recent change somewhere exposed a latent bug between LRA and the definition
> of the movsi_compare0 pattern.
> 
> This pattern ties the source and destination register of a set together
> a (match_dup) and register constraints:
> 
>    [(set (reg:CC CC_REGNUM)
> 	(compare:CC (match_operand:SI 1 "s_register_operand" "0,r")
> 		    (const_int 0)))
>     (set (match_operand:SI 0 "s_register_operand" "=r,r")
> 	(match_dup 1))]
> 
> This confuses LRA which expects the source and destination register of
> a set to be different.
> 
> reduced.c: In function '_IO_vfscanf_internal':
> reduced.c:104:1: internal compiler error: in lra_create_copy, at lra.c:1512
>      }
>  ^
> 0x8c3f9a lra_create_copy(int, int, int)
> 	/work/gcc-dev/src/gcc/gcc/lra.c:1512
> 0x8e4ab0 process_bb_lives
> 	/work/gcc-dev/src/gcc/gcc/lra-lives.c:568
> 0x8e4ab0 lra_create_live_ranges(bool)
> 	/work/gcc-dev/src/gcc/gcc/lra-lives.c:1019
> 0x8c5a39 lra(_IO_FILE*)
> 	/work/gcc-dev/src/gcc/gcc/lra.c:2356
> 0x873a96 do_reload
> 	/work/gcc-dev/src/gcc/gcc/ira.c:5415
> 0x873a96 execute
> 	/work/gcc-dev/src/gcc/gcc/ira.c:5576
> Please submit a full bug report,
> 
> We can fix the pattern by moving away from match_dup and using register
> tying with constraints consistently.
> 
> I'm not entirely convinced that this is legitimate (my vague recollection is
> that register tying should only be used to tie inputs to outputs).
> 
> This has passed testing on a bunch of ARM targets, and fixes the build
> issues I've been seeing.

Looking at this too, as an LRA exercise. I don't really think the
pattern is wrong, rather LRA should just avoid creating the copy in this
case; it's a result of operand constraining, after all.

Attached is the small LRA patch, pending testing. Vladimir should weight
in on this.

Thanks,
Chung-Lin

	* ira-lives.c (process_bb_lives): Skip creating copy during
	insn sca when src/dest has constrained to same regno.

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