work around constant folding bug 61144 in gcc 4.9.0 and 4.9.1

previously we detected this bug in configure and issued advice for a
workaround, but this turned out not to work. since then gcc 4.9.0 has
appeared in several distributions, and now 4.9.1 has been released
without a fix despite this being a wrong code generation bug which is
supposed to be a release-blocker, per gcc policy.

since the scope of the bug seems to affect only data objects (rather
than functions) whose definitions are overridable, and there are only
a very small number of these in musl, I am just changing them from
const to volatile for the time being. simply removing the const would
be sufficient to make gcc 4.9.1 work (the non-const case was
inadvertently fixed as part of another change in gcc), and this would
also be sufficient with 4.9.0 if we forced -O0 on the affected files
or on the whole build. however it's cleaner to just remove all the
broken compiler detection and use volatile, which will ensure that
they are never constant-folded. the quality of a non-broken compiler's
output should not be affected except for the fact that these objects
are no longer const and thus possibly add a few bytes to data/bss.

this change can be reconsidered and possibly reverted at some point in
the future when the broken gcc versions are no longer relevant.

configure

@@ -495,27 +495,6 @@ printf "no\n"
 fail "$0: error: unsupported long double type"
 fi
 
-#
-# Check for known bug in GCC 4.9.0 that results in a broken libc.
-#
-if test "$cc_is_gcc" = yes ; then
-printf "checking for gcc constant folding bug with weak aliases... "
-echo 'static int x = 0;' > "$tmpc"
-echo 'extern int y __attribute__((__weak__, __alias__("x")));' >> "$tmpc"
-echo 'extern int should_appear;' >> "$tmpc"
-echo 'int foo() { return y ? should_appear : 0; }' >> "$tmpc"
-case "$($CC $CFLAGS_C99FSE -I./arch/$ARCH -I./include \
-  $CPPFLAGS $CFLAGS_AUTO $CFLAGS -S -o - "$tmpc" 2>/dev/null)" in
-*should_appear*)
-printf "no\n"
-;;
-*)
-printf "yes\n"
-fail "$0: error: broken compiler; try CFLAGS=-fno-toplevel-reorder"
-;;
-esac
-fi
-
 printf "creating config.mak... "
 
 cmdline=$(quote "$0")

src/stdio/__stdio_exit.c

 #include "stdio_impl.h"
 
-static FILE *const dummy_file = 0;
+static FILE *volatile dummy_file = 0;
 weak_alias(dummy_file, __stdin_used);
 weak_alias(dummy_file, __stdout_used);
 weak_alias(dummy_file, __stderr_used);

src/stdio/fflush.c

@@ -19,7 +19,7 @@ static int __fflush_unlocked(FILE *f)
 }
 
 /* stdout.c will override this if linked */
-static FILE *const dummy = 0;
+static FILE *volatile dummy = 0;
 weak_alias(dummy, __stdout_used);
 
 int fflush(FILE *f)

src/stdio/stderr.c

@@ -13,4 +13,4 @@ static FILE f = {
 	.lock = -1,
 };
 FILE *const stderr = &f;
-FILE *const __stderr_used = &f;
+FILE *volatile __stderr_used = &f;

src/stdio/stdin.c

@@ -12,4 +12,4 @@ static FILE f = {
 	.lock = -1,
 };
 FILE *const stdin = &f;
-FILE *const __stdin_used = &f;
+FILE *volatile __stdin_used = &f;

src/stdio/stdout.c

@@ -13,4 +13,4 @@ static FILE f = {
 	.lock = -1,
 };
 FILE *const stdout = &f;
-FILE *const __stdout_used = &f;
+FILE *volatile __stdout_used = &f;

src/thread/pthread_create.c

@@ -116,12 +116,12 @@ static int start(void *p)
 #define ROUND(x) (((x)+PAGE_SIZE-1)&-PAGE_SIZE)
 
 /* pthread_key_create.c overrides this */
-static const size_t dummy = 0;
+static volatile size_t dummy = 0;
 weak_alias(dummy, __pthread_tsd_size);
-static void *const dummy_tsd[1] = { 0 };
+static void *dummy_tsd[1] = { 0 };
 weak_alias(dummy_tsd, __pthread_tsd_main);
 
-static FILE *const dummy_file = 0;
+static FILE *volatile dummy_file = 0;
 weak_alias(dummy_file, __stdin_used);
 weak_alias(dummy_file, __stdout_used);
 weak_alias(dummy_file, __stderr_used);

src/thread/pthread_key_create.c

 #include "pthread_impl.h"
 
-const size_t __pthread_tsd_size = sizeof(void *) * PTHREAD_KEYS_MAX;
+volatile size_t __pthread_tsd_size = sizeof(void *) * PTHREAD_KEYS_MAX;
 void *__pthread_tsd_main[PTHREAD_KEYS_MAX] = { 0 };
 
 static void (*keys[PTHREAD_KEYS_MAX])(void *);