From a0ae2ba37ca479c6edddec8634b25686be965e0d Mon Sep 17 00:00:00 2001 From: Peter Korsgaard Date: Mon, 27 Aug 2018 22:50:57 +0200 Subject: [PATCH] bn_mul.h: fix x86 PIC inline ASM compilation with GCC < 5 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 --- include/mbedtls/bn_mul.h | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-) diff --git a/include/mbedtls/bn_mul.h b/include/mbedtls/bn_mul.h index b587317d9593..74a2d29bed60 100644 --- a/include/mbedtls/bn_mul.h +++ b/include/mbedtls/bn_mul.h @@ -50,13 +50,29 @@ #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( \