[Bug c/57259] New: extra load and store with -fexcess-precision=standard

[nszabolcs at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57259

            Bug ID: 57259
           Summary: extra load and store with -fexcess-precision=standard
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nszabolcs at gmail dot com
            Target: i686-pc-linux-gnu

with -fexcess-precision=standard extra load and store is generated
on i386 linux even when it is not needed
(the code is correct but does unnecessary work on all optimization levels)

in the following example volatile enforces the store of x+y
and is correctly compiled with default flags, but with
-fexcess-precision=standard a further fstps/flds pair is generated

(note that using volatile to force the evaluation of an expression
to modify fenv as a side-effect is common idiom in code that uses
fenv, such code also needs standard excess-precision handling)

$ cat example.c
void f(float x, float y)
{
        volatile float z = x+y;
}
$ gcc-4.8 -S -fno-asynchronous-unwind-tables -fomit-frame-pointer example.c
$ cat example.s
        .file   "example.c"
        .text
        .globl  f
        .type   f, @function
f:
        subl    $16, %esp
        flds    20(%esp)
        fadds   24(%esp)
        fstps   12(%esp)
        addl    $16, %esp
        ret
        .size   f, .-f
        .ident  "GCC: (GNU) 4.8.0"
        .section        .note.GNU-stack,"",@progbits
$ gcc-4.8 -S -fno-asynchronous-unwind-tables -fomit-frame-pointer
-fexcess-precision=standard example.c
$ cat example.s
        .file   "example.c"
        .text
        .globl  f
        .type   f, @function
f:
        subl    $20, %esp
        flds    24(%esp)
        flds    28(%esp)
        faddp   %st, %st(1)
        fstps   (%esp)
        flds    (%esp)
        fstps   16(%esp)
        addl    $20, %esp
        ret
        .size   f, .-f
        .ident  "GCC: (GNU) 4.8.0"
        .section        .note.GNU-stack,"",@progbits