https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101301
--- Comment #1 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
Here's the assembly for the switch statement with -O3:
cmpl $55555, %edi
je .L6
jle .L18
movl $8, %eax
cmpl $88888, %edi
je .L1
jle .L19
xorl %eax, %eax
cmpl $99999, %edi
sete %al
leal (%rax,%rax,8), %eax
ret
.p2align 4,,10
.p2align 3
.L18:
movl $3, %eax
cmpl $33333, %edi
je .L1
jle .L20
xorl %eax, %eax
cmpl $44444, %edi
sete %al
sall $2, %eax
ret
.p2align 4,,10
.p2align 3
.L19:
movl $6, %eax
cmpl $66666, %edi
je .L1
xorl %eax, %eax
movl $7, %edx
cmpl $77777, %edi
cmove %edx, %eax
ret
.p2align 4,,10
.p2align 3
.L6:
movl $5, %eax
.L1:
ret
.p2align 4,,10
.p2align 3
.L20:
movl $1, %eax
cmpl $11111, %edi
je .L1
xorl %eax, %eax
cmpl $22222, %edi
sete %al
addl %eax, %eax
ret
and here for the if statement version:
cmpl $55554, %edi
jle .L2
cmpl $77776, %edi
jle .L3
cmpl $77777, %edi
je .L6
cmpl $88888, %edi
je .L7
xorl %eax, %eax
cmpl $99999, %edi
sete %al
leal (%rax,%rax,8), %eax
ret
.p2align 4,,10
.p2align 3
.L3:
cmpl $55555, %edi
je .L9
xorl %eax, %eax
movl $6, %edx
cmpl $66666, %edi
cmove %edx, %eax
ret
.p2align 4,,10
.p2align 3
.L2:
cmpl $33332, %edi
jle .L5
cmpl $33333, %edi
je .L11
xorl %eax, %eax
cmpl $44444, %edi
sete %al
sall $2, %eax
ret
.p2align 4,,10
.p2align 3
.L5:
movl $1, %eax
cmpl $11111, %edi
je .L1
xorl %eax, %eax
cmpl $22222, %edi
sete %al
addl %eax, %eax
ret
.p2align 4,,10
.p2align 3
.L11:
movl $3, %eax
.L1:
ret
.p2align 4,,10
.p2align 3
.L7:
movl $8, %eax
ret
.p2align 4,,10
.p2align 3
.L9:
movl $5, %eax
ret
.p2align 4,,10
.p2align 3
.L6:
movl $7, %eax
ret
Stepping through the code with a debugger, here is a table on the number
of conditional jumps for all special values and in all relevant intervals:
5000 5 3
11111 5 3
15000 5 3
22222 5 3
25000 5 3
33333 3 3
35000 4 3
44444 4 3
50000 4 3
55555 1 3
60000 5 3
66666 5 3
70000 5 3
77777 5 3
80000 5 4
88888 3 4
90000 4 4
99999 4 4
100000 4 4
If one assumes each case as equally likely, the number of conditional jumps
goes down from 81 to 62.
Assuming each case statement is reached once, the total number of
conditional jumps executed goes from 35 to 29.
