Thanks again Samuel Neves: the "gcc bug" is actually undefined behavior caused by strict aliasing. For now just passing -fno-strict-aliasing, maybe unions to fix it later

Makefile

@@ -26,7 +26,7 @@ WARNFLAGS = -pedantic -Wall -Wextra -Werror -Wunreachable-code \
 	 
 	 
 INCFLAGS = -Isrc/include -Iinclude -Isrc/$(ARCH)
-LANGFLAGS = -std=c99
+LANGFLAGS = -std=c99 -fno-strict-aliasing
 GENFLAGS = -ffunction-sections -fdata-sections -fvisibility=hidden -fomit-frame-pointer -fPIC
 OFLAGS = -O3
 

src/arch_x86_64/p448.h

@@ -89,21 +89,11 @@ p448_is_zero (
     const p448_t *in
 );
   
-static
-#ifndef GCC_HAS_A_BUG_SO_DONT_INLINE_FIELD_BIAS
-    __inline__
-#endif
-void
+static __inline__ void
 p448_bias (
     p448_t *inout,
     int amount
-)
-#ifdef GCC_HAS_A_BUG_SO_DONT_INLINE_FIELD_BIAS
-    __attribute__((unused,noinline,optimize("O1")))
-#else
-    __attribute__((unused,always_inline))
-#endif
-;
+) __attribute__((unused,always_inline));
          
 void
 p448_mul (

src/scalarmul.c

@@ -1,10 +1,6 @@
 /* Copyright (c) 2014 Cryptography Research, Inc.
  * Released under the MIT License.  See LICENSE.txt for license information.
  */
-#if (defined(__GNUC__) && !defined(__clang__) && defined(__x86_64__))
-  /* This works around an apparent compiler bug in GCC, thanks Samuel Neves */
-  #define GCC_HAS_A_BUG_SO_DONT_INLINE_FIELD_BIAS
-#endif
 
 #include "word.h"