Per VognsenFeb 23
Saw this when randomly flipping through the code base of something posted to Lobsters. C or C++ is not the language for you if you think you can write code like that. https://c.godbolt.org/z/8q5zoh6Ke
Compiler Explorer - C (x86-64 gcc 15.2)

// https://github.com/abdimoallim/jit typedef int32_t i32; typedef uint8_t u8; typedef size_t usz; typedef struct { u8* buf; usz len; } jit_buf; void jit_ensure(jit_buf* j, usz n) {} // bad void jit_emit_i32(jit_buf* j, i32 v) { jit_ensure(j, 4); j->buf[j->len + 0] = (u8)(v); j->buf[j->len + 1] = (u8)(v >> 8); j->buf[j->len + 2] = (u8)(v >> 16); j->buf[j->len + 3] = (u8)(v >> 24); j->len += 4; } // good void jit_emit_i32_fixed(jit_buf* j, i32 v) { jit_ensure(j, 4); u8* buf = j->buf; usz len = j->len; buf[len + 0] = (u8)(v); buf[len + 1] = (u8)(v >> 8); buf[len + 2] = (u8)(v >> 16); buf[len + 3] = (u8)(v >> 24); j->len = len + 4; } int main() {}

Show threadPer VognsenFeb 23
There's a certain systems language with pervasive noalias guarantees where you can write code like that but C is assuredly not that language.
Show threadSean BarrettFeb 24
@pervognsen Isn't this one of the cases that strict aliasing optimizations are supposed to enable?
Show threadPer VognsenFeb 24
@nothings Not in this case. A store via a char pointer is expressly allowed to alias any type.
Show threadMarc B. ReynoldsFeb 24

@pervognsen @nothings Both GCC & clang produce the desired if 'restrict' is added to the jit_buf pointer. That seems "wrong" to me.

https://c.godbolt.org/z/6sGsh7xnY

Compiler Explorer - C

// https://github.com/abdimoallim/jit typedef int32_t i32; typedef uint8_t u8; typedef size_t usz; typedef struct { u8* buf; usz len; } jit_buf; void jit_ensure(jit_buf*j, usz n) {} // bad void jit_emit_i32(jit_buf* j, i32 v) { jit_ensure(j, 4); j->buf[j->len + 0] = (u8)(v); j->buf[j->len + 1] = (u8)(v >> 8); j->buf[j->len + 2] = (u8)(v >> 16); j->buf[j->len + 3] = (u8)(v >> 24); j->len += 4; } // seems wrong to me void jit_emit_i32_humm(jit_buf* restrict j, i32 v) { jit_ensure(j, 4); j->buf[j->len + 0] = (u8)(v); j->buf[j->len + 1] = (u8)(v >> 8); j->buf[j->len + 2] = (u8)(v >> 16); j->buf[j->len + 3] = (u8)(v >> 24); j->len += 4; } // good void jit_emit_i32_fixed(jit_buf* j, i32 v) { jit_ensure(j, 4); u8* buf = j->buf; usz len = j->len; buf[len + 0] = (u8)(v); buf[len + 1] = (u8)(v >> 8); buf[len + 2] = (u8)(v >> 16); buf[len + 3] = (u8)(v >> 24); j->len = len + 4; } int main() {}

Show threadPer VognsenFeb 24
@mbr @nothings No? That's as expected.
Show threadSean BarrettFeb 24
@pervognsen @mbr The char pointer inside the struct "inherits" the restrict qualifier from the pointer to the struct? I didn't realize that (I think I've never actually used restrict.)
Show threadPaul Khuong
@nothings @pervognsen @mbr the char buf doesn't alias with the struct because the pointer to the struct is restricted.
Feb 24 at 12:28pmWeb
Show threadSean BarrettFeb 24
@pkhuong @pervognsen Ah right, of course, that makes sense. Thanks!