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bn_mul.h: fix x86 PIC inline ASM compilation with GCC < 5

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Fixes #1910

With ebx added to the MULADDC_STOP clobber list to fix #1550, the inline
assembly fails to build with GCC < 5 in PIC mode with the following error:

include/mbedtls/bn_mul.h:46:13: error: PIC register clobbered by ‘ebx’ in ‘asm’

This is because older GCC versions treated the x86 ebx register (which is
used for the GOT) as a fixed reserved register when building as PIC.

This is fixed by an improved register allocator in GCC 5+.  From the release
notes:

Register allocation improvements: Reuse of the PIC hard register, instead of
using a fixed register, was implemented on x86/x86-64 targets.  This
improves generated PIC code performance as more hard registers can be used.

https://www.gnu.org/software/gcc/gcc-5/changes.html

As a workaround, detect this situation and disable the inline assembly,
similar to the MULADDC_CANNOT_USE_R7 logic.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
This commit is contained in:
Peter Korsgaard 2018-08-27 22:50:57 +02:00 committed by Tom Cosgrove
parent d5b1eb51db
commit c0546e351f
1 changed files with 17 additions and 1 deletions
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18
library/bn_mul.h
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@ -90,13 +90,29 @@
#if defined(__GNUC__) && \
( !defined(__ARMCC_VERSION) || __ARMCC_VERSION >= 6000000 )
/*
* GCC < 5.0 treated the x86 ebx (which is used for the GOT) as a
* fixed reserved register when building as PIC, leading to errors
* like: bn_mul.h:46:13: error: PIC register clobbered by 'ebx' in 'asm'
*
* This is fixed by an improved register allocator in GCC 5+. From the
* release notes:
* Register allocation improvements: Reuse of the PIC hard register,
* instead of using a fixed register, was implemented on x86/x86-64
* targets. This improves generated PIC code performance as more hard
* registers can be used.
*/
#if defined(__GNUC__) && __GNUC__ < 5 && defined(__PIC__)
#define MULADDC_CANNOT_USE_EBX
#endif
/*
* Disable use of the i386 assembly code below if option -O0, to disable all
* compiler optimisations, is passed, detected with __OPTIMIZE__
* This is done as the number of registers used in the assembly code doesn't
* work with the -O0 option.
*/
#if defined(__i386__) && defined(__OPTIMIZE__)
#if defined(__i386__) && defined(__OPTIMIZE__) && !defined(MULADDC_CANNOT_USE_EBX)
#define MULADDC_X1_INIT \
{ mbedtls_mpi_uint t; \