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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/103829] New: [9/10/11/12 Regression] missing
 shrink wrapping for simple/obvious code
Date: Sat, 25 Dec 2021 22:16:17 +0000
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X-Bugzilla-Component: rtl-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103829

            Bug ID: 103829
           Summary: [9/10/11/12 Regression] missing shrink wrapping for
                    simple/obvious code
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---
            Target: x86_64-linux-gnu aarch64-linux-gnu

Take:

extern int _IO_getc (void *) ;
extern int *__errno_location (void) __attribute__ ((__const__));
void readError ( void );
typedef
   struct {
      void* handle;
      int buffer;
      int buffLive;
      char mode;
   }
   BitStream;

int bsGetBit ( BitStream* bs )
{=20=20
   if (bs->buffLive > 0) {
      bs->buffLive --;
      return ( ((bs->buffer) >> (bs->buffLive)) & 0x1 );
   } else {
      int retVal =3D _IO_getc (bs->handle);
      if ( retVal =3D=3D (-1) ) {
         if ((*__errno_location ()) !=3D 0) readError();
         return 2;
      }
      bs->buffLive =3D 7;
      bs->buffer =3D retVal;
      return ( ((bs->buffer) >> 7) & 0x1 );
   }
}
----- CUT ----
GCC 4.9.0-8.5.0 was able to shrink wrap the above function.
But starting in GCC 9, GCC does not; there is an extra mov using a callee s=
aved
register.
This happens on both aarch64 and x86_64 So I suspect it was a generic change
which caused it.=