1.15. SWITCH()/CASE/DEFAULT
The last instruction,CMP R0, #2, is needed to check ifa= 2.
If it is not true, thenADRNEloads a pointer to the string«something unknown \n»intoR0, sinceahas
already been checked to be equal to 0 or 1, and we can sure that theavariable is not equal to these
numbers at this point. And ifR0 = 2, a pointer to the string«two\n»will be loaded byADREQintoR0.
ARM: Optimizing Keil 6/2013 (Thumb mode)
.text:000000D4 f1:
.text:000000D4 10 B5 PUSH {R4,LR}
.text:000000D6 00 28 CMP R0, #0
.text:000000D8 05 D0 BEQ zero_case
.text:000000DA 01 28 CMP R0, #1
.text:000000DC 05 D0 BEQ one_case
.text:000000DE 02 28 CMP R0, #2
.text:000000E0 05 D0 BEQ two_case
.text:000000E2 91 A0 ADR R0, aSomethingUnkno ; "something unknown\n"
.text:000000E4 04 E0 B default_case
.text:000000E6 zero_case: ; CODE XREF: f1+4
.text:000000E6 95 A0 ADR R0, aZero ; "zero\n"
.text:000000E8 02 E0 B default_case
.text:000000EA one_case: ; CODE XREF: f1+8
.text:000000EA 96 A0 ADR R0, aOne ; "one\n"
.text:000000EC 00 E0 B default_case
.text:000000EE two_case: ; CODE XREF: f1+C
.text:000000EE 97 A0 ADR R0, aTwo ; "two\n"
.text:000000F0 default_case ; CODE XREF: f1+10
.text:000000F0 ; f1+14
.text:000000F0 06 F0 7E F8 BL __2printf
.text:000000F4 10 BD POP {R4,PC}
As was already mentioned, it is not possible to add conditional predicates to most instructions in Thumb
mode, so the Thumb-code here is somewhat similar to the easily understandable x86CISC-style code.
ARM64: Non-optimizing GCC (Linaro) 4.9
.LC12:
.string "zero"
.LC13:
.string "one"
.LC14:
.string "two"
.LC15:
.string "something unknown"
f12:
stp x29, x30, [sp, -32]!
add x29, sp, 0
str w0, [x29,28]
ldr w0, [x29,28]
cmp w0, 1
beq .L34
cmp w0, 2
beq .L35
cmp w0, wzr
bne .L38 ; jump to default label
adrp x0, .LC12 ; "zero"
add x0, x0, :lo12:.LC12
bl puts
b .L32
.L34:
adrp x0, .LC13 ; "one"
add x0, x0, :lo12:.LC13
bl puts