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From: "tmsriram at google dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/61599] New: [x86_64] With -mcmodel=medium, extern global arrays without size are not treated conservatively.
Date: Tue, 24 Jun 2014 17:32:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61599

            Bug ID: 61599
           Summary: [x86_64] With -mcmodel=medium, extern global arrays
                    without size are not treated conservatively.
           Product: gcc
           Version: 4.10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tmsriram at google dot com
                CC: davidxl at google dot com, ppluzhnikov at google dot com

foo.cc
======

char c[1*1024*1024*1024];

extern int bar();
int main()
{
  return bar() + c[225];
}

bar.cc
======
extern char c[];

int bar()
{
  return c[225];
}


$ g++ -mcmodel=medium foo.cc bar.cc -fdata-sections
BFD linker warns:
bar.cc:(.text+0x7): relocation truncated to fit: R_X86_64_PC32 against symbol
`c' defined in .lbss.c section in foo.o

Reason is the compiler does not treat 'c' conservatively as being in .lbss when
it does not know its size. Worse, adding -mlarge-data-threshold=0 still does
not solve the problem.

Changing the declaration "extern char c[]" to "extern char c[1*1024*1024*1024]"
solves the problem.