[PATCH] Fix libbacktrace zdebug decompression on big endian (PR other/85161)

[Jakub Jelinek]

Hi!

As mentioned in the PR, GCC (and clang) predefines
{__BYTE_ORDER__,__ORDER_{LITTLE,BIG,PDP}_ENDIAN__}
macros, and {,sys/,machine/}endian.h headers predefine
{,__}{BYTE_ORDER,{LITTLE,BIG,PDP}_ENDIAN}
macros (depending on which target and feature test macros).
elf.c in GCC 8 used __BYTE_ORDER, which is endian.h macro, but
didn't include that header and it on glibc just happened to be included
indirectly because of default feature test macros from stdlib.h,
and used non-existing __ORDER_BIG_ENDIAN macro; as __BYTE_ORDER is always
non-zero when defined (1234, 4321 etc.), that means __builtin_bswap32
was never used.

The following patch just uses the GCC/clang predefined macros if known to be
big or little endian, and otherwise just falls back to portable code (that
good compilers can still optimize).

Bootstrapped/regtested on x86_64-linux and i686-linux and tested on
powerpc64-linux, ok for trunk?

2018-04-03  Jakub Jelinek  <jakub@redhat.com>

	PR other/85161
	* elf.c (elf_zlib_fetch): Fix up predefined macro names in test for
	big endian, only use 32-bit loads if endianity macros are predefined
	and indicate big or little endian.

--- libbacktrace/elf.c.jj	2018-02-15 12:30:53.948579969 +0100
+++ libbacktrace/elf.c	2018-04-03 14:47:32.536550472 +0200
@@ -1086,12 +1086,19 @@ elf_zlib_fetch (const unsigned char **pp
       return 0;
     }
 
+#if defined(__BYTE_ORDER__) && defined(__ORDER_LITTLE_ENDIAN__) \
+    && defined(__ORDER_BIG_ENDIAN__) \
+    && (__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__ \
+        || __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__)
   /* We've ensured that PIN is aligned.  */
   next = *(const uint32_t *)pin;
 
-#if __BYTE_ORDER == __ORDER_BIG_ENDIAN
+#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
   next = __builtin_bswap32 (next);
 #endif
+#else
+  next = pin[0] | (pin[1] << 8) | (pin[2] << 16) | (pin[3] << 24);
+#endif
 
   val |= (uint64_t)next << bits;
   bits += 32;

	Jakub