On Freitag, 14. August 2020 18:43:12 CEST Stefan Kanthak wrote:
> Hi @ll,
>
> in his ACM queue article <https://queue.acm.org/detail.cfm?id=3372264>,
> Matt Godbolt used the function
>
> | bool isWhitespace(char c)
> | {
> |
> | return c == ' '
> |
> | || c == '\r'
> | || c == '\n'
> | || c == '\t';
> |
> | }
>
> as an example, for which GCC 9.1 emits the following assembly for AMD64
>
> processors (see <https://godbolt.org/z/acm19_conds>):
> | xor eax, eax ; result = false
> | cmp dil, 32 ; is c > 32
> | ja .L4 ; if so, exit with false
> | movabs rax, 4294977024 ; rax = 0x100002600
> | shrx rax, rax, rdi ; rax >>= c
> | and eax, 1 ; result = rax & 1
> |
> |.L4:
> | ret
>
No it doesn't. As your example shows if you took the time to read it, it is
what gcc emit when generating code to run on a _haswell_ architecture. If you
remove -march=haswell from the command line you get:
xor eax, eax
cmp dil, 32
ja .L1
movabs rax, 4294977024
mov ecx, edi
shr rax, cl
and eax, 1
It uses one mov more, but no shrx.
'Allan
