From c0546e351f6d7ab50eb1de8cef1d0d167760fccc Mon Sep 17 00:00:00 2001
From: Peter Korsgaard <peter@korsgaard.com>
Date: Mon, 27 Aug 2018 22:50:57 +0200
Subject: [PATCH] bn_mul.h: fix x86 PIC inline ASM compilation with GCC < 5
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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.

Upstream-Status: Backport [https://github.com/Mbed-TLS/mbedtls/commit/c0546e351f6d7ab50eb1de8cef1d0d167760fccc]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
---
 library/bn_mul.h | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

--- a/third_party/openthread/repo/third_party/mbedtls/repo/include/mbedtls/bn_mul.h
+++ b/third_party/openthread/repo/third_party/mbedtls/repo/include/mbedtls/bn_mul.h
@@ -55,12 +55,28 @@
     ( !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(                                    \