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[ARM/gold] Fix the "DIV usage mismatch between blah.o and output" error

Ian Lance Taylor iant@google.com
Wed Jan 9 15:27:00 GMT 2013 
On Wed, Dec 19, 2012 at 11:32 AM, Ben Cheng <bccheng@google.com> wrote:
> Gas in binutils 2.21 used to aggressively set the Tag_DIV_use flag to
> 1 (ie "Not allowed"), which will be propagated through the attribute
> handling code in bfd/elf32-arm.c and gold/arm.cc and serve as the big
> hammer to set Tag_DIV_use to 1 in the final output.
>
> In 2.22 changes were made into gas and bfd to correctly allow 0 (ie
> "Allowed in Thumb-ISA, v7-R or v7-M") and 2 (ie "Allowed in v7-A with
> integer division extension") to co-exist, but gold is not picking up
> the corresponding fix and it still strictly expects identical
> Tag_DIV_use values across all binary files or at least a 1 to unify
> the attribute.
>
> Following are two simple test functions used to reproduce the problem:
>
> div.c:
> int div(int i, int j)
> {
>     return i/j;
> }
>
> main.c:
> include <stdio.h>
>
> extern int div(int, int);
> int main()
> {
>     printf("4 / 2 = %d\n", div(4, 2));
>     return 0;
> }
>
> If div.c is compiled with -mcpu=cortex-a15 and main.c is compiled with
> -mcpu=cortex-a9, only div.o will have Tag_DIV_use set to 2 and main.o
> will use the default value 0, but gold will fail to link them with the
> DIV usage mismatch error.
>
> This patch basically ports the new logic from bfd/elf32_arm.c over to
> gold/arm.cc and with it the linker is able to link the program.

Sorry for the slow review.

Please send patches in diff -u or diff -p format.  The default diff
format is hard to read and hard to apply to the sources.

Use // comments.  For a block in a switch, put the opening brace after
the case line.

I committed your patch as follows.

Thanks.

Ian


2013-01-09  Ben Cheng  <bccheng@google.com>

	* arm.cc (Target_arm::attributes_accept_div): New function.
	(Target_arm::attributes_forbid_div): New function.
	(Target_arm::merge_object_attributes): Merge the Tag_DIV_use
	attribute using the same new functions as what bfd/elf32_arm.c
	does.
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