https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87485
--- Comment #19 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Vladimir Makarov from comment #18)
> (In reply to Vladimir Makarov from comment #17)
> > I've reproduced it. Clearly, it is some bug in LRA conflict calculation.
> > I will be working on it.
>
> I investigated it more. Before scheduling we have
>
> (insn 60 122 61 7 (parallel [
> (set (subreg:DI (reg:TI 125) 0)
> (zero_extend:DI (udiv:SI (reg:SI 116)
> (reg:SI 119))))
> (set (reg:SI 118)
> (umod:SI (reg:SI 116)
> (reg:SI 119)))
> (clobber (reg:CC 17 flags))
> ...
>
> (insn 90 69 92 7 (set (reg/i:SI 0 ax)
> (const_int 0 [0])) "pr87485.c":34:1 67 {*movsi_internal}
> (nil))
>
> After scheduling we have
>
> (insn 90 54 60 7 (set (reg/i:SI 0 ax)
> (const_int 0 [0])) "pr87485.c":34:1 67 {*movsi_internal}
> (nil))
> (insn 60 90 61 7 (parallel [
> (set (subreg:DI (reg:TI 125) 0)
> (zero_extend:DI (udiv:SI (reg:SI 116)
> (reg:SI 119))))
> (set (reg:SI 118)
> (umod:SI (reg:SI 116)
> (reg:SI 119)))
> (clobber (reg:CC 17 flags))
This happens when return pair gets split in expand_function_end. sched1 pass
has certain code that marks both instructions as CANT_MOVE (c.f. a comment in
add_branch_dependencies:
Insns setting TARGET_CLASS_LIKELY_SPILLED_P registers (usually return
values) are not moved before reload because we can wind up with register
allocation failures.
)
stack protect epilogue code gets inserted in between return pair, so the final
assignment to a hard register gets away from the USE insn. The above code is
ineffective in case of split return pair.
So, expand_function_end should be fixed to emit an assignment to a function
value hard reg just before use insn. Some parts of the compiler (sched1 and
mode switching passes) depend on an unbroken return pair!
