Re: [Patch, lra] PR70751, correct the cost for spilling non-pseudo into memory

[Dominik Vogt <vogt at linux dot vnet dot ibm dot com>]

On Thu, Jun 09, 2016 at 09:52:39AM -0600, Jeff Law wrote:
> On 06/08/2016 10:47 AM, Jiong Wang wrote:
> >As discussed on the PR
> >
> >  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70751,
> >
> >here is the patch.

This commit has introduced an ICE with s390x, march=z13.  Is it a
backend bug or one in the middleend?

-- x.c --
void foo(int *n, int off, int *m)
{
  int i;

  for(i = 0 ;i <= 3; i++)
    {
      n[off + i] = m[2 * i];
      n[off + 7 - i] = m[2 * i + 1];
    }
}
-- snip --

  $ gcc -S -O3 -march=z13 x.c                                   
x.c: In function âfooâ:
x.c:10:1: internal compiler error: Max. number of generated reload insns per insn is achieved (90)

 }
 ^
0x804c34b7 lra_constraints(bool)
	gcc/lra-constraints.c:4468
0x804b085f lra(_IO_FILE*)
	gcc/lra.c:2290
0x804690fb do_reload
	gcc/ira.c:5384
0x804690fb execute
	gcc/ira.c:5568

> >
> >For this particular failure on arm, *arm_movsi_insn has the following
> >operand
> >constraints:
> >    operand 0: "=rk,r,r,r,rk,m"
> >  operand 1: "rk, I,K,j,mi,rk"
> >
> >gcc won't explicitly refuse an unmatch CT_MEMORY operand (r235184) if it
> >comes from substituion that alternative (alt) 4 got a chance to compete
> >with
> >alt 0, and eventually be the winner as it's with rld_nregs=0 while alt 0 is
> >with rld_nregs=1.
> >
> >I fell it's OK to give alt 4 a chance here, but we should calculate the
> >cost
> >correctly.
> >
> >For alt 4, it should be treated as spill into memory, but currently lra
> >only
> >recognize a spill for pseudo register.   While the spilled rtx for alt 4
> >is a
> >plus after equiv substitution.
> >
> >         (plus:SI (reg/f:SI 102 sfp)
> >            (const_int 4 [0x4]))
> >
> >This patch thus let lra-constraint cost spill of Non-pseudo as well and
> >fixed
> >the regression.
> >
> >x86_64/aarch64/arm boostrap and regression OK.
> >arm bootstrapped cc1 is about 0.3% smaller in code size.
> >
> >OK for trunk?
> >
> >gcc/
> >        PR rtl-optimization/70751
> >        * lra-constraints.c (process_alt_operands): Recognize Non-pseudo
> >spilled into
> >        memory.
> >
> I think the change itself is fine, but the comment could use some
> work. Actually I think you just need to remove the first sentence
> (the one referring to BZ70751 and r235184) and keep everything from
> "Suppose a target" onward.
> 
> OK with that fix.
> 
> jeff
> 


Ciao

Dominik ^_^  ^_^

-- 

Dominik Vogt
IBM Germany