1From a0ae2ba37ca479c6edddec8634b25686be965e0d Mon Sep 17 00:00:00 2001
2From: Peter Korsgaard <peter@korsgaard.com>
3Date: Mon, 27 Aug 2018 22:50:57 +0200
4Subject: [PATCH] bn_mul.h: fix x86 PIC inline ASM compilation with GCC < 5
5MIME-Version: 1.0
6Content-Type: text/plain; charset=UTF-8
7Content-Transfer-Encoding: 8bit
8
9Fixes #1910
10
11With ebx added to the MULADDC_STOP clobber list to fix #1550, the inline
12assembly fails to build with GCC < 5 in PIC mode with the following error:
13
14include/mbedtls/bn_mul.h:46:13: error: PIC register clobbered by ‘ebx’ in ‘asm’
15
16This is because older GCC versions treated the x86 ebx register (which is
17used for the GOT) as a fixed reserved register when building as PIC.
18
19This is fixed by an improved register allocator in GCC 5+.  From the release
20notes:
21
22Register allocation improvements: Reuse of the PIC hard register, instead of
23using a fixed register, was implemented on x86/x86-64 targets.  This
24improves generated PIC code performance as more hard registers can be used.
25
26https://www.gnu.org/software/gcc/gcc-5/changes.html
27
28As a workaround, detect this situation and disable the inline assembly,
29similar to the MULADDC_CANNOT_USE_R7 logic.
30
31Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
32Upstream: https://github.com/ARMmbed/mbedtls/pull/1986
33---
34 include/mbedtls/bn_mul.h | 18 +++++++++++++++++-
35 1 file changed, 17 insertions(+), 1 deletion(-)
36
37diff --git a/include/mbedtls/bn_mul.h b/include/mbedtls/bn_mul.h
38index b587317d9..74a2d29be 100644
39--- a/include/mbedtls/bn_mul.h
40+++ b/include/mbedtls/bn_mul.h
41@@ -50,13 +50,29 @@
42 #if defined(__GNUC__) && \
43     ( !defined(__ARMCC_VERSION) || __ARMCC_VERSION >= 6000000 )
44
45+/*
46+ * GCC < 5.0 treated the x86 ebx (which is used for the GOT) as a
47+ * fixed reserved register when building as PIC, leading to errors
48+ * like: bn_mul.h:46:13: error: PIC register clobbered by ‘ebx’ in ‘asm’
49+ *
50+ * This is fixed by an improved register allocator in GCC 5+. From the
51+ * release notes:
52+ * Register allocation improvements: Reuse of the PIC hard register,
53+ * instead of using a fixed register, was implemented on x86/x86-64
54+ * targets. This improves generated PIC code performance as more hard
55+ * registers can be used.
56+ */
57+#if defined(__GNUC__) && __GNUC__ < 5 && defined(__PIC__)
58+#define MULADDC_CANNOT_USE_EBX
59+#endif
60+
61 /*
62  * Disable use of the i386 assembly code below if option -O0, to disable all
63  * compiler optimisations, is passed, detected with __OPTIMIZE__
64  * This is done as the number of registers used in the assembly code doesn't
65  * work with the -O0 option.
66  */
67-#if defined(__i386__) && defined(__OPTIMIZE__)
68+#if defined(__i386__) && defined(__OPTIMIZE__) && !defined(MULADDC_CANNOT_USE_EBX)
69
70 #define MULADDC_INIT                        \
71     asm(                                    \
72--
732.11.0
74
75