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Speed up is_ascii_hex_digit #479

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Closed

Speed up is_ascii_hex_digit #479

wants to merge 1 commit into from

Commits on Aug 22, 2023

  1. Speed up is_ascii_hex_digit 

    It's possible to reduce the number of required comparisons by converting `c` into lowercase, so that instead of checking both lowercase and uppercase ranges, only lowercase range needs to be checked.

This allows clang to generate branchless x86_64 asm (https://compiler-explorer.com/z/P6f7c8e4T). Instead of
```
is_ascii_hex_digit(char):                # @is_ascii_hex_digit(char)
        lea     ecx, [rdi - 48]
        mov     al, 1
        cmp     cl, 10
        jb      .LBB0_3
        lea     ecx, [rdi - 65]
        cmp     cl, 6
        jb      .LBB0_3
        add     dil, -97
        cmp     dil, 6
        setb    al
.LBB0_3:
        ret
```
clang generates
```
is_ascii_hex_digit(char):                # @is_ascii_hex_digit(char)
        lea     eax, [rdi - 48]
        cmp     al, 10
        setb    cl
        or      dil, 32
        add     dil, -97
        cmp     dil, 6
        setb    al
        or      al, cl
        ret
```
that is 2 instructions shorter and contains no branches.
    @ttsugriy
    ttsugriy authored Aug 22, 2023
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