GCC Bugzilla Bug 36467

From AVR Freaks forum:
http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&p=453770#453770

Adding an offset to a pointer to a structure. If the structure size is 16, the
generated code is a loop with shifts. If the structure size is 17, the
generated code uses MUL* instructions and generates shorter code than when the
size is 16.

Command lines:
avr-gcc -save-temps -Os -mmcu=atmega32 -c test.c -o test.o
avr-gcc -save-temps -Os -mmcu=atmega32 -c test2.c -o test2.o

See the code generation of funct().

Test cases to follow...

Created an attachment (id=15734) [edit]
Test case with structure size == 16.

Created an attachment (id=15735) [edit]
Test case with structure size == 17.

Generated code when structure size is 16 (test.i):

funct:
/* prologue: function */
/* frame size = 0 */
        lds r24,head
        mov r30,r24
        clr r31
        sbrc r30,7
        com r31
        ldi r24,4
1:      lsl r30
        rol r31
        dec r24
        brne 1b
        subi r30,lo8(-(qq))
        sbci r31,hi8(-(qq))
        ld r24,Z
        sbrc r24,1
        std Z+1,__zero_reg__
.L3:
        ret


Generated code when structure size is 17 (test2.i):

funct:
/* prologue: function */
/* frame size = 0 */
        lds r24,head
        ldi r25,lo8(17)
        muls r24,r25
        movw r30,r0
        clr r1
        subi r30,lo8(-(qq))
        sbci r31,hi8(-(qq))
        ld r24,Z
        sbrc r24,1
        std Z+1,__zero_reg__
.L3:
        ret

It makes sense in one respect
We don't have fast shift by 4 bits and code defaults to loop for Os. Seems we
should be selective as MUL is indeed shorter.

Though I think gcc may be confused by our poor cost data and perhaps was alsp
mislead into using shift instead of MUL.