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From: "robert.suchanek at imgtec dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/65862] New: [MIPS] IRA/LRA issue: integers spilled to floating-point registers
Date: Thu, 23 Apr 2015 16:04:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65862

            Bug ID: 65862
           Summary: [MIPS] IRA/LRA issue: integers spilled to
                    floating-point registers
           Product: gcc
           Version: 5.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: robert.suchanek at imgtec dot com
                CC: matthew.fortune at imgtec dot com, vmakarov at redhat dot com,
                    wdijkstr at arm dot com

Following up the following thread:
https://gcc.gnu.org/ml/gcc/2015-04/msg00239.html

Here is a reduced testcase from the Linux kernel:

$ cat sort.c
int a, c;
int *b;
void
fn1(int p1, int *p2(void *, void *), void *p3(void *, void *, int)) {
  int n = c;
  for (;;) {
    a = 1;
    for (; a < n;) {
      p1 && p2(0, (int *) (p1 + 1));
      p3(0, b + p1, 0);
    }
  }
}

Spill/reload to/from FP reg should be triggerable with (tested on SVN rev.
222257):
$ mips-img-linux-gnu -mips32r6 -O2 sort.c 

Because of ALL_REGS assigned to most of allocnos, LRA uses FP regs freely. The
class is preferred because of the equal cost between registers and memory. This
likely happened because of the following fix:

2011-12-20  Vladimir Makarov  <vmakarov@redhat.com>                  

    PR target/49865
    * ira-costs.c (find_costs_and_classes): Prefer registers even 
      if the memory cost is the same.                            

As Matthew already pointed out, one way to prevent this is through increasing
the cost of moving between GP and FP registers for integral modes.

I briefly tested out Wilco's patch but it did not appear to have the same
effect as changing the cost and I've seen a few ICEs when building the kernel.