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From: "hubicka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/103541] New: unnecessary spills around const
 functions calls
Date: Fri, 03 Dec 2021 15:16:13 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103541

            Bug ID: 103541
           Summary: unnecessary spills around const functions calls
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hubicka at gcc dot gnu.org
  Target Milestone: ---

While looking into reasons why modref causes some code size increases I not=
iced
that we produce unnecessary spill on x86-64 here:

float a;

__attribute__((const)) float foo (float);

float
test()
{
        return a + foo(a) + a;
}

we load "a" into register and then spill it to stack because all SSE regist=
ers
are clobbered by the call. This seems to happen somewhere between gcc 4.1 a=
nd
4.6.  It is caused by:

          /* We can combine a reg def from one insn into a reg use in
             another over a call if the memory is readonly or the call
             const/pure.  However, we can't set reg_equiv notes up for
             reload over any call.  The problem is the equivalent form
             may reference a pseudo which gets assigned a call
             clobbered hard reg.  When we later replace REG with its
             equivalent form, the value in the call-clobbered reg has
             been changed and all hell breaks loose.  */
          ret =3D valid_combine;
          if (!MEM_READONLY_P (memref)
              && !RTL_CONST_OR_PURE_CALL_P (insn))
            return valid_none;

in ira.c:validate_equiv_mem

If I read the comment correctly it is afraid of the address of memory readi=
ng
being altered by the call (using call clobbered registers). But here it is a
constant, so perhaps we can just rule this out when MEM rtx does not mention
registers or does not mention any callee clobbered registers?=