c compiler warnings silently ignored, giving wrong results/undefined behavior

[timotheecour]

After codegen, c compiler warnings are silently ignored; this can give undefined behavior in which I ran into.

Example

# extracted from system/timers
proc mach_absolute_time(): int64 {.importc, header: "<mach/mach.h>".}
echo mach_absolute_time()

Current Output

the output is random stack garbage:

1158149750 # 1st run
-170359460 # 2nd run

Expected Output

356630733360154 # first run
356632381452769 # first run

Possible Solution

don't ignore warnings, and even error on warnings (except maybe a whitelist).
When compiling with --listCmd we get:

clang -c  -w  -I/Users/timothee/.choosenim/toolchains/nim-0.20.0/lib -I/Users/timothee/git_clone/nim/timn/tests/nim/all -o /tmp/d01bc/t04377.nim.c.o /tmp/d01bc/t04377.nim.c

if we remove -w we get:

/tmp/d01bc/t04377.nim.c:149:8: warning: implicit declaration of function 'mach_absolute_time' is invalid in C99 [-Wimplicit-function-declaration]
        T2_ = mach_absolute_time();

that warning currently gets ignored without error or any indication of which, and the generated code produces garbage.

The 2nd problem here is that the header is wrong (and also the return type is wrong): it should be:

proc mach_absolute_time(): uint64 {.importc, header: "<mach/mach_time.h>".}

with this, it gives the expected output.

Note: the reason system/timers actually works is by luck: it's because MachTimebaseInfoData itself uses the right header; but minor modifications would re-exhibit that bug, eg if we comment out an unrelated part of the code (eg mach_timebase_info(timeBaseInfo)), that correct header would get optimized out, and the undefined behavior would be there.

Note: this bug is obviously not specific to system/timers and can occur in many other unrelated settings

Additional Information

• Was it working in the previous Nim releases?
  no

nim --version
Nim Compiler Version 0.20.0 [MacOSX: amd64]
Compiled at 2019-06-06
Copyright (c) 2006-2019 by Andreas Rumpf

git hash: e7471cebae2a404f3e4239f199f5a0c422484aac
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