https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103829
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 ) { if (bs->buffLive > 0) { bs->buffLive --; return ( ((bs->buffer) >> (bs->buffLive)) & 0x1 ); } else { int retVal = _IO_getc (bs->handle); if ( retVal == (-1) ) { if ((*__errno_location ()) != 0) readError(); return 2; } bs->buffLive = 7; bs->buffer = 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 saved register. This happens on both aarch64 and x86_64 So I suspect it was a generic change which caused it.