Reproduction of an optimizer bug with Visual Studio 2017

repro.c

/* Repro optimizer issue with Visual Studio 2017 */
/* @language: c99 */
/*

# Visual Studio 2015 (working)
--
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\bin\amd64\cl.exe

cl.exe -Fe:repro.exe repro.c -nologo -FC -EHsc -Z7 -Oi -GR- -Gm- -O2 -Z7 -W4 -WX -DEBUG

TEST	repro.exe
Event layout:
           int_count: [0x08,0x09)
          int_values: [0x0c,0x18)
       int_values[0]: [0x0c,0x10)
       int_values[1]: [0x10,0x14)
---
Event:
       00 01 02 03 04 05 06 07  08 09 0a 0b 0c 0d 0e 0f
0x0000 20 00 00 00 00 00 00 00  01 CD CD CD 2A 00 00 00
0x0010 CD CD CD CD CD CD CD CD
---
exited with code 0

0xCD is memory that wasn't touched.
int_count is 0x01 (1 argument)
int_values[0]: is 0x2A, which is our sentinel value

# Visual Studio 2017 (broken)

D:\Visual Studio 2017\VC\Tools\MSVC\14.10.25017\bin\HostX64\x64\cl.exe
cl.exe -Fe:repro.exe -nologo -FC -EHsc -Z7 -Oi -GR- -Gm- -O2 -Z7  -W4 -WX -DEBUG repro.c

TEST	repro.exe
Event layout:
           int_count: [0x08,0x09)
          int_values: [0x0c,0x18)
       int_values[0]: [0x0c,0x10)
       int_values[1]: [0x10,0x14)
---
Event:
       00 01 02 03 04 05 06 07  08 09 0a 0b 0c 0d 0e 0f
0x0000 20 00 00 00 00 00 00 00  01 CD CD CD CD CD CD CD
0x0010 2A 00 00 00 CD CD CD CD 
---
Assertion failed: event->int_values[0] == 42, file repro.c, line 111
ERROR: Test  failed
exited with code -1073740791

0xCD is memory that wasn't touched.
int_count is 0x01 (1 argument, correct)
int_values[0]: is 0xCD. Our sentinel value has been put instead at int_values[1]!

*/

#include <assert.h>
#include <stddef.h>
#include <stdio.h>

struct SPDR_Event {
    unsigned long long ts_ticks;
    char int_count;
    int int_values[3];
};

static int equal_zstring(char const* a, char const *b) { return strcmp(a, b) == 0; }

static void print_hexdump(unsigned char* bytes, size_t bytes_n, unsigned int offset_base);

int main(int argc, char** argv)
{
    (void) argc;
    (void) argv;

    /* print record layout */ {
        printf("Event layout:\n");
#define OP(m) printf("%20s: [0x%02x,0x%02x)\n", #m, \
        (unsigned int) offsetof(struct SPDR_Event, m), \
        (unsigned int) (offsetof(struct SPDR_Event, m) + \
        sizeof ((struct SPDR_Event*)0)->m))
        OP(int_count);
        OP(int_values);
        OP(int_values[0]);
        OP(int_values[1]);
#undef OP
        printf("---\n");
        fflush(stdout);
    }

    int clock_tick = 32;
    struct SPDR_Event event_on_stack;
    /* create event */ {
        memset(&event_on_stack, 0xCD, sizeof event_on_stack);
        {
            event_on_stack.ts_ticks = clock_tick++;
            event_on_stack.int_count = 0;
        }
        {
            int i = event_on_stack.int_count++;
            event_on_stack.int_values[i] = 42;
        }
    }

    /* inspect event afterwards */ {
        struct SPDR_Event* event = &event_on_stack;
        printf("Event:\n");
        print_hexdump((unsigned char*)event, sizeof *event, 0);
        printf("---\n");
        fflush(stdout);
        assert(event->int_count == 1);
        assert(event->int_values[0] == 42);
    }
    return 0;
}

static void print_hexdump(unsigned char* bytes, size_t bytes_n, unsigned int offset_base)
{
    unsigned char* bytes_l = bytes + bytes_n;
    unsigned int row_offset = offset_base;
    printf("      ");
    {
        unsigned int n = 0;
        while(n < 8) {
            printf(" %02x", n);
            ++n;
        }
        printf(" ");
        while(n < 16) {
            printf(" %02x", n);
            ++n;
        }
        printf("\n");
    }
    while (bytes != bytes_l) {
        int n = 8;
        printf("0x%04x", row_offset);
        while (n-- && bytes != bytes_l) {
            printf(" %02X", *bytes); ++bytes;
        }
        printf(" ");
        n = 8;
        while (n-- && bytes != bytes_l) {
            printf(" %02X", *bytes); ++bytes;
        }
        printf ("\n");
        row_offset += 16;
    }
}

@uucidl: this has been fixed in the latest release, i.e. Visual Studio 2017 version 15.3.1 (released on August 18, 2017). I tried specifically with that compiler:

D:\dev>where cl
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.11.25503\bin\HostX64\x64\cl.exe

D:\dev>cl
Microsoft (R) C/C++ Optimizing Compiler Version 19.11.25506 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

usage: cl [ option... ] filename... [ /link linkoption... ]