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

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merged 2 commits into from
Jul 21, 2022
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Backport 2.28: bn_mul.h: fix x86 PIC inline ASM compilation with GCC < 5 #6096

merged 2 commits into from
Jul 21, 2022

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Backport of #1986

jacmet and others added 2 commits July 19, 2022 09:02
@jacmet @tom-cosgrove-arm
bn_mul.h: fix x86 PIC inline ASM compilation with GCC < 5
9d9d45c 
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>
@tom-cosgrove-arm
Add a ChangeLog entry
accd50d 
Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
@tom-cosgrove-arm tom-cosgrove-arm marked this pull request as ready for review July 19, 2022 08:05
@tom-cosgrove-arm tom-cosgrove-arm mentioned this pull request Jul 19, 2022
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@tom-cosgrove-arm tom-cosgrove-arm added needs-review Every commit must be reviewed by at least two team members, needs-ci Needs to pass CI tests needs-reviewer This PR needs someone to pick it up for review bug component-crypto Crypto primitives and low-level interfaces labels Jul 19, 2022
@daverodgman daverodgman removed needs-ci Needs to pass CI tests needs-review Every commit must be reviewed by at least two team members, needs-reviewer This PR needs someone to pick it up for review labels Jul 20, 2022
@daverodgman daverodgman added needs-review Every commit must be reviewed by at least two team members, priority-low Low priority - this may not receive review soon labels Jul 20, 2022
mpg
mpg approved these changes Jul 21, 2022
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LGTM

@mpg mpg added approved Design and code approved - may be waiting for CI or backports and removed needs-review Every commit must be reviewed by at least two team members, labels Jul 21, 2022
@daverodgman daverodgman merged commit 5048045 into Mbed-TLS:mbedtls-2.28 Jul 21, 2022
@tobil4sk tobil4sk mentioned this pull request Mar 11, 2024
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