Bug#808471: Multiple -march flags

[Yuri D'Elia <wavexx@thregr.org>]

Package: gcc-5
Version: 5.3.1-3
Severity: normal

I couldn't find anything on the matter, but what happens if -march is repeated
more than once?  I would assume the usual behavior that the last flag "wins".

On a haswell host, the following:

gcc -march=native -march=opteron

or

gcc -march=opteron -march=native

both emit code which is illegal for the opteron ISA, as if -march=native had
special treatment. Specifying -march=opteron alone works as intended.

Since I generally override the default flags in makefiles by appending
exceptions where needed, I'd sort of expected the regular behavior.

Is this intended?

Thanks.

-- System Information:
Debian Release: stretch/sid
  APT prefers unstable
  APT policy: (900, 'unstable'), (800, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 4.3.0-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages gcc-5 depends on:
ii  binutils      2.25.90.20151209-1
ii  cpp-5         5.3.1-3
ii  gcc-5-base    5.3.1-3
ii  libc6         2.21-4
ii  libcc1-0      5.3.1-3
ii  libgcc-5-dev  5.3.1-3
ii  libgcc1       1:5.3.1-3
ii  libgmp10      2:6.1.0+dfsg-2
ii  libisl15      0.15-3
ii  libmpc3       1.0.3-1
ii  libmpfr4      3.1.3-2
ii  libstdc++6    5.3.1-3
ii  zlib1g        1:1.2.8.dfsg-2+b1

Versions of packages gcc-5 recommends:
ii  libc6-dev  2.21-4

Versions of packages gcc-5 suggests:
ii  gcc-5-doc         5.2.0-1
pn  gcc-5-locales     <none>
pn  gcc-5-multilib    <none>
pn  libasan2-dbg      <none>
pn  libatomic1-dbg    <none>
pn  libcilkrts5-dbg   <none>
pn  libgcc1-dbg       <none>
pn  libgomp1-dbg      <none>
pn  libitm1-dbg       <none>
pn  liblsan0-dbg      <none>
pn  libmpx0-dbg       <none>
pn  libquadmath0-dbg  <none>
pn  libtsan0-dbg      <none>
pn  libubsan0-dbg     <none>