[gem5-dev] Gem5 on MacOS/clang --as-needed flag detection

[Rodrigues, Arun F via gem5-dev]

Hello,

I’m running into some issues building libgem5 on MacOS using clang. The build 
compiles correctly, but fails during linking because the LLVM linker doesn’t 
understand the –as-needed flag. It seems that gem5 is incorrectly passing this 
flag.

Buildline:
scons build/RISCV/libgem5_opt.dylib -j 8 --without-tcmalloc

Failure:
[  SHLINK]  -> RISCV/libgem5_opt.dylib
ld: unknown option: --as-needed
clang: error: linker command failed with exit code 1 (use -v to see invocation)
scons: *** [build/RISCV/libgem5_opt.dylib] Error 1

The toplevel SConstruct file, line 343 has:

conf.CheckLinkFlag('-Wl,--as-needed')

On MacOS with clang, this check returns ‘no’ since the clang linker doesn’t 
take the –as-needed flag. But, it seems that CheckLinkFlag()’s default behavior 
(set_for_shared=True) seems to be to append the flag, even if the config test 
fails.

Configure.py:

def CheckLinkFlag(context, flag, autoadd=True, set_for_shared=True):
    context.Message("Checking for linker %s support... " % flag)
    last_linkflags = context.env['LINKFLAGS']
    context.env.Append(LINKFLAGS=[flag])
    ret = context.TryLink('int main(int, char *[]) { return 0; }', '.cc')
    if not (ret and autoadd):
        context.env['LINKFLAGS'] = last_linkflags
    if set_for_shared:
        assert(autoadd)
        context.env.Append(SHLINKFLAGS=[flag])
    context.Result(ret)
    return ret

This results in the linking phase failing on MacOS.

I was able to get it to build by changing the toplevel SConstruct to:
   conf.CheckLinkFlag('-Wl,--as-needed', True, False)

I’m not too familiar with the –as-needed flag and why it is needed, so I’m 
wondering:

  *   Does this fix seem reasonable?
  *   Is the CheckLinkFlag() behavior correct? Should the flag be appended to 
SHLINKFLAGS even if the test fails?

Additional info:

  *   Clang/clang++ Apple clang version 11.0.0 (clang-1100.0.33.17)
  *   Uname -a: Darwin s1033894 19.6.0 Darwin Kernel Version 19.6.0: Sun Jul  5 
00:43:10 PDT 2020; root:xnu-6153.141.1~9/RELEASE_X86_64 x86_64


Thanks,

arun



Thanks,

arun



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