http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50448
Bug #: 50448 Summary: [avr] Missed optimization accessing struct component with known, absolute address. Classification: Unclassified Product: gcc Version: 4.6.1 Status: UNCONFIRMED Keywords: missed-optimization Severity: normal Priority: P3 Component: target AssignedTo: unassig...@gcc.gnu.org ReportedBy: g...@gcc.gnu.org CC: eric.wedding...@atmel.com Target: avr typedef struct { unsigned char a,b,c,d; } SPI_t; #define SPIE (*(SPI_t volatile*) 0x0AC0) void foo (void) { SPIE.d = 0xAA; while (!(SPIE.c & 0x80)); SPIE.d = 0xBB; while (!(SPIE.c & 0x80)); } avr-gcc-4.6.1 -Os -S -fdump-tree-optimized -fdump-rtl-expand compiles that code to foo: ldi r24,lo8(-86) ldi r30,lo8(2752) ldi r31,hi8(2752) std Z+3,r24 .L2: lds r24,2754 sbrs r24,7 rjmp .L2 ldi r24,lo8(-69) ldi r30,lo8(2752) ldi r31,hi8(2752) std Z+3,r24 .L3: lds r24,2754 sbrs r24,7 rjmp .L3 ret Instead of loading the address 2752 two times, it's sufficient to load it once or to do a direct access to 2755 and avoid loading the constant altogether. The load appeard first in .expand; .optimized looks fine: foo () { signed char D.1932; volatile unsigned char D.1931; signed char D.1930; volatile unsigned char D.1929; <bb 2>: MEM[(volatile struct SPI_t *)2752B].d ={v} 170; <bb 3>: D.1929_3 ={v} MEM[(volatile struct SPI_t *)2752B].c; D.1930_4 = (signed char) D.1929_3; if (D.1930_4 >= 0) goto <bb 3>; else goto <bb 4>; <bb 4>: MEM[(volatile struct SPI_t *)2752B].d ={v} 187; <bb 5>: D.1931_7 ={v} MEM[(volatile struct SPI_t *)2752B].c; D.1932_8 = (signed char) D.1931_7; if (D.1932_8 >= 0) goto <bb 5>; else goto <bb 6>; <bb 6>: return; }