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

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

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9d9d45c
bn_mul.h: fix x86 PIC inline ASM compilation with GCC < 5
jacmet Aug 27, 2018
accd50d
Add a ChangeLog entry
tom-cosgrove-arm Jul 19, 2022
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4 changes: 4 additions & 0 deletions ChangeLog.d/bn_mul-fix-x86-pic-compilation-for-gcc-4.txt
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@@ -0,0 +1,4 @@
Bugfix
* Fix a long-standing build failure when building x86 PIC code with old
gcc (4.x). The code will be slower, but will compile. We do however
recommend upgrading to a more recent compiler instead. Fixes #1910.
18 changes: 17 additions & 1 deletion include/mbedtls/bn_mul.h
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Expand Up @@ -94,13 +94,29 @@
#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_INIT \
asm( \
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