Posted on February 20, 2016 at 11:34
I simply created enough context to compile a function with your code.
When I compile with -O0 I get:
myisr:
@ Function supports interworking.
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 1, uses_anonymous_args = 0
@ link register save eliminated.
str fp, [sp, #-4]!
add fp, sp, #0
ldr r2, .L2
ldr r3, .L2+4
ldrh r3, [r3]
mov r3, r3, asl #16
mov r3, r3, lsr #16
strh r3, [r2] @ movhi
ldr r3, .L2
ldr r2, .L2+8
ldrb r2, [r2] @ zero_extendqisi2
strb r2, [r3]
ldr r3, .L2
ldr r2, .L2+8
ldrb r2, [r2, #1] @ zero_extendqisi2
strb r2, [r3]
ldr r3, .L2
ldr r2, .L2+8
ldrb r2, [r2, #2] @ zero_extendqisi2
strb r2, [r3]
ldr r3, .L2
ldr r2, .L2+8
ldrb r2, [r2, #3] @ zero_extendqisi2
strb r2, [r3]
ldr r3, .L2
ldr r2, .L2+8
ldrb r2, [r2, #4] @ zero_extendqisi2
strb r2, [r3]
ldr r3, .L2
ldr r2, .L2+8
ldrb r2, [r2, #5] @ zero_extendqisi2
strb r2, [r3]
sub sp, fp, #0
@ sp needed
ldr fp, [sp], #4
bx lr
While when I compile with -O2 I much better optimized code:
myisr:
@ Function supports interworking.
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
ldr r3, .L2
ldr r2, .L2+4
ldrh r1, [r3]
ldrb r2, [r2, #5] @ zero_extendqisi2
ldr r3, .L2+8
strh r1, [r3, #8] @ movhi
strb r2, [r3, #8]
bx lr
However it is no longer correct.... (because you omitted the ''volatile'' keyword, the compiler is allowed to optimize unnecessary writes away).
