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From: "peter at peterzhu dot ca" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/102452] New: Structs with flexible array members are not
 optimized on stack
Date: Wed, 22 Sep 2021 13:22:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102452

            Bug ID: 102452
           Summary: Structs with flexible array members are not optimized
                    on stack
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: peter at peterzhu dot ca
  Target Milestone: ---

Created attachment 51494
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D51494&action=3Dedit
test.c

Structs with flexible array members are not optimized on the stack. Testing
with the test.c file shown below (which outputs the runtime in seconds), we=
 see
that the compiled program is significantly slower when the struct has a
flexible array member than when it does not. This was tested on GCC 9.3.0 a=
nd
10.3.0 on Ubuntu 20.04.

$ gcc -O3 test.c
$ ./a.out
0.302769
$ gcc -O3 -DUSE_FLEX_ARR=3D1 test.c
$ ./a.out
0.728760

clang does not have this issue.

$ clang -O3 test.c
$ ./a.out
0.312194
$ clang -O3 -DUSE_FLEX_ARR=3D1 test.c
$ ./a.out
0.301175

This is what test.c looks like:

#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <string.h>

struct Test {
    long is_a;
    union {
        struct {
            long one;
            long two;
            long three;
        } a;
        struct {
            int one;
            int two;
            int three;
            int four;
#if USE_FLEX_ARR
            char arr[];
#endif
        } b;
    } as;
};

#define COUNT 100000000

static inline struct Test make_test_a(struct Test *test)
{
    if (test->is_a) {
        return *test;
    } else {
        struct Test ret;
        ret.as.a.one =3D test->as.b.one;
        ret.as.a.two =3D test->as.b.two;
        ret.as.a.three =3D test->as.b.three;
        return ret;
    }
}

/* This function should be optimized to not allocate struct Test on the sta=
ck
 * since it only uses attribute "three". */
static inline long get_three(struct Test *test)
{
    return make_test_a(test).as.a.three;
}

int main(int argc, char *argv[])
{
    struct timespec start, end;

    struct Test *mem =3D malloc(sizeof(struct Test) * COUNT);
    memset(mem, 0, sizeof(struct Test) * COUNT);

    clock_gettime(CLOCK_MONOTONIC, &start);
    {
        for (int i =3D 0; i < COUNT; i++) {
            long three =3D get_three(&mem[i]);
            if (three) {
                /* Impossible case. */
                printf("what\n");
            }
        }
    }
    clock_gettime(CLOCK_MONOTONIC, &end);

    double time =3D (end.tv_sec - start.tv_sec) + (end.tv_nsec - start.tv_n=
sec) /
1000000000.0;
    printf("%f\n", time);

    return 0;
}=