[llvm-bugs] [Bug 50682] New: Clang doesn't honor unroll #pragma to disable loop unrolling

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50682

            Bug ID: 50682
           Summary: Clang doesn't honor unroll #pragma to disable loop
                    unrolling
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: kobalicek.petr at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

Clang doesn't honor unroll #pragma to disable loop unrolling. I tried the
following 3 approaches to disable loop unrolling, but it's never honored. The
code is always autovectorized and unrolled regardless of #pragmas used.

Is there any way to make clang not unroll loops? Because in many cases I know
the loops will have very few iterations and I mark them with compiler specific
pragmas to not unroll, however, clang doesn't really honor them.

Compile flags:
  -O2 -fno-math-errno -mavx2 -std=c++2a

The problem happens with -O2, -O3, and even -Os.

Sample code:

#include <stdint.h>

uint32_t countBits1(uint32_t* x, uint32_t n) {
    uint32_t count = 0;
    #pragma clang loop unroll(disable)
    for (uint32_t i = 0; i < n; i++) {
        count += __builtin_popcount(x[i]);
    }
    return count;
}

uint32_t countBits2(uint32_t* x, uint32_t n) {
    uint32_t count = 0;
    #pragma nounroll
    for (uint32_t i = 0; i < n; i++) {
        count += __builtin_popcount(x[i]);
    }
    return count;
}

uint32_t countBits3(uint32_t* x, uint32_t n) {
    uint32_t count = 0;
    #pragma unroll(1)
    for (uint32_t i = 0; i < n; i++) {
        count += __builtin_popcount(x[i]);
    }
    return count;
}

Compile explorer:
  https://godbolt.org/z/EcoGqoaYG

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