clang loop optimizer pragma warns on rasterio.cpp.

[schwehr]

What is the bug?

rasterio.cpp triggers a complaint with UBSAN mode with recent clang (like in the last couple weeks), but as this is a custom environment, I don't know if this happens with older clangs. I've seen pass-failed happen a few years ago with non-gdal code.

#if defined(__clang__)
#pragma clang loop vectorize(disable)
#endif

gcore/rasterio.cpp:5954:5: error: loop not vectorized: the optimizer was unable to perform the requested transformation; the transformation might be disabled or specified as part of an unsupported transformation ordering [-Werror,-Wpass-failed=transform-warning]
 5954 |     for (size_t i = 0; i < nIters; ++i)
      |     ^
gcore/rasterio.cpp:6049:5: error: loop not vectorized: the optimizer was unable to perform the requested transformation; the transformation might be disabled or specified as part of an unsupported transformation ordering [-Werror,-Wpass-failed=transform-warning]
 6049 |     for (size_t i = 0; i < nIters; ++i)
      |     ^
2 errors generated.

Not sure why the loop doesn't unroll.

Steps to reproduce the issue

I don't have great details on how to reproduce. Sorry.

Versions and provenance

Hit this GDAL from head on 2025-01-10.

Additional context

Not sure the best way to disable this in the GDAL code base and is that reasonable to do? Locally, I set -Wno-pass-failed for now and then the UBSAN run goes just fine.

I looked at CPL_NOSANITIZE_UNSIGNED_INT_OVERFLOW and maybe this is correct __attribute__((no_sanitize_undefined))?

I will investigate more.

[rouault]

Reproduced locally with quite ancient clang.
Easiest fix is to suppress the warning

[rouault]

Fix in #11646