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Backport 2.28: bn_mul.h: fix x86 PIC inline ASM compilation with GCC < 5 #6096

Merged
merged 2 commits into from
Jul 21, 2022
Merged

Backport 2.28: bn_mul.h: fix x86 PIC inline ASM compilation with GCC < 5 #6096

merged 2 commits into from
Jul 21, 2022

Commits on Jul 19, 2022

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

    Fixes Mbed-TLS#1910

With ebx added to the MULADDC_STOP clobber list to fix Mbed-TLS#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>
    @jacmet @tom-cosgrove-arm
    jacmet authored and tom-cosgrove-arm committed Jul 19, 2022
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  2. Add a ChangeLog entry 

    Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
    @tom-cosgrove-arm
    tom-cosgrove-arm committed Jul 19, 2022
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