dev2 does not compile on arm32

[Noxybot]

Seems like a regression introduced in 2.1.8 with this commit: a964322.

The error I'm getting:

src/segment-map.c:42:35: error: variable length array declaration not allowed at file scope
static _Atomic(mi_segmap_part_t*) mi_segment_map[MI_SEGMENT_MAP_MAX_PARTS]; // = { NULL, .. }

MI_SEGMENT_MAP_MAX_PARTS seems to be expanded to this:

((((4294967295U))/((8*((1<<(2))*(1024UL)-128))*((1UL<<(7+(13+(2)))))))+1)

LMK if you need more information.

[daanx]

Strange -- I wonder if it has to do with the _Atomic type since the length seems just a constant expression?

[Noxybot]

Daan (@daanx) it’s a constant expression, but it seems to be expanded to some gigantic value (bigger than we can have static array). So maybe some overflow/underflow. When I tried pasting it to godbolt (clang19/armv7), it seemed like the denominator is zero…

[daanx]

Thanks -- I think I fixed it :-)

[Noxybot]

Seems to compile now. Can we cherry-pick it to dev2?

[Noxybot]

Actually, still does not compile, now I'm getting this (cherry-picked the commit to dev2):

src/segment-map.c:55:51: error: division by zero is undefined [-Werror,-Wdivision-by-zero]
  const uintptr_t segindex = ((uintptr_t)segment) / MI_SEGMENT_MAP_PART_SPAN;

[daanx]

I think I fixed the issue now :-). Strange it all worked on Win32 -- it must be something in how the preprocessors evaluates constants. Let me know how it goes.

[Noxybot]

With the latest commit we're back to the original error. I've taken a look at the code, and it seems like we divide bytes by bits...
Also, the reason why it works on dev and does not work on dev2 is that segment size changed: on dev2 segment is 4mb on 32bit, so the denominator overflows enumerator.

PR to fix this potential issue : #1020.

It helps me to compile mimalloc on arm32 without any overflow in preprocessor.

[daanx]

Thanks for your help! I looked again at the code and your PR. However, the logic seems right as each bit covers a segment. -- but I misspelled a macro name in the preprocessor condition. I think it is now fixed -- fingers crossed.
(btw. the max parts should be 2 on 32-bit since each part has almost 8 K bits and thus covers 8k * segment_size memory.)

[Noxybot]

with those changes dev2 compiles on arm32, thanks!