35 ms·

`s[0] == 'h'` isn't sufficient to guarantee that `s[3]` can be access without a segfault, so the compiler is not allowed to perform this optimization. If you u

by ynik 10mo ago

`s[0] == 'h'` isn't sufficient to guarantee that `s[3]` can be access without a segfault, so the compiler is not allowed to perform this optimization. If you use `&` instead of `&&` (so that all array elements are accessed unconditionally), the optimization will happen: https://godbolt.org/z/KjdT16Kfb https://godbolt.org/z/KjdT16Kfb (also note you got the endianness wrong in your hand-optimized version)

abainbridge 10mo ago
Ooo, I'd never thought of using & like that. Interesting. > (also note you got the endianness wrong in your hand-optimized version) Doh :-)
- rdc12 10mo ago
  Matt Godbolt's talk on ray tracers, shows how effective that change can be. Think it was that talk anyway. https://www.youtube.com/watch?v=HG6c4Kwbv4I https://www.youtube.com/watch?v=HG6c4Kwbv4I
NooneAtAll3 10mo ago
good ol' short circuiting
zrm 10mo ago
> If you use `&` instead of `&&` (so that all array elements are accessed unconditionally), the optimization will happen But then you're accessing four elements of a string that could have a strlen of less than 3. If the strlen is 1 then the short circuit case saves you because s[1] will be '\0' instead of 'e' and then you don't access elements past the end of the string. The "optimized" version is UB for short strings.
- Denvercoder9 10mo ago
  Yes, so that's why the compiler can't and doesn't emit the optimized version if you write the short circuited version - because it behaves differently for short strings.
- immibis 10mo ago
  UB doesn't exist in the processor (it does, but not here). If the compiler knows the pointer is aligned it can do the transformation.
  - zrm 10mo ago
    For the compiler to know the pointer is aligned it would have to actually be aligned and there is no guarantee that it is.
kragen 10mo ago
This is fantastic, thanks! This is the approach I use in httpdito to detect the CRLFCRLF that terminates an HTTP/1.0 GET request, but I'm doing it in assembly.

by ynik 10mo ago

abainbridge 10mo ago

Ooo, I'd never thought of using & like that. Interesting. > (also note you got the endianness wrong in your hand-optimized version) Doh :-)

rdc12 10mo ago
Matt Godbolt's talk on ray tracers, shows how effective that change can be. Think it was that talk anyway. https://www.youtube.com/watch?v=HG6c4Kwbv4I https://www.youtube.com/watch?v=HG6c4Kwbv4I

NooneAtAll3 10mo ago

good ol' short circuiting

zrm 10mo ago

> If you use `&` instead of `&&` (so that all array elements are accessed unconditionally), the optimization will happen But then you're accessing four elements of a string that could have a strlen of less than 3. If the strlen is 1 then the short circuit case saves you because s[1] will be '\0' instead of 'e' and then you don't access elements past the end of the string. The "optimized" version is UB for short strings.

Denvercoder9 10mo ago
Yes, so that's why the compiler can't and doesn't emit the optimized version if you write the short circuited version - because it behaves differently for short strings.
immibis 10mo ago
UB doesn't exist in the processor (it does, but not here). If the compiler knows the pointer is aligned it can do the transformation.
- zrm 10mo ago
  For the compiler to know the pointer is aligned it would have to actually be aligned and there is no guarantee that it is.

kragen 10mo ago

This is fantastic, thanks! This is the approach I use in httpdito to detect the CRLFCRLF that terminates an HTTP/1.0 GET request, but I'm doing it in assembly.