bypass indirection through pointer objects to access stdin/out/err

by ABI, the public stdin/out/err macros use extern pointer objects,
and this is necessary to avoid copy relocations that would be
expensive and make the size of the FILE structure part of the ABI.
however, internally it makes sense to access the underlying FILE
objects directly. this avoids both an indirection through the GOT to
find the address of the stdin/out/err pointer objects (which can't be
computed PC-relative because they may have been moved to the main
program by copy relocations) and an indirection through the resulting
pointer object.

in most places this is just a minor optimization, but in the case of
getchar and putchar (and the unlocked versions thereof), ipa constant
propagation makes all accesses to members of stdin/out PC-relative or
GOT-relative, possibly reducing register pressure as well.

src/include/stdio.h

@@ -0,0 +1,18 @@
+#ifndef STDIO_H
+#define STDIO_H
+
+#include "../../include/stdio.h"
+
+#undef stdin
+#undef stdout
+#undef stderr
+
+extern hidden FILE __stdin_FILE;
+extern hidden FILE __stdout_FILE;
+extern hidden FILE __stderr_FILE;
+
+#define stdin (&__stdin_FILE)
+#define stdout (&__stdout_FILE)
+#define stderr (&__stderr_FILE)
+
+#endif

src/stdio/stderr.c

@@ -1,7 +1,9 @@
 #include "stdio_impl.h"
 
+#undef stderr
+
 static unsigned char buf[UNGET];
-static FILE f = {
+hidden FILE __stderr_FILE = {
 	.buf = buf+UNGET,
 	.buf_size = 0,
 	.fd = 2,
@@ -12,5 +14,5 @@ static FILE f = {
 	.close = __stdio_close,
 	.lock = -1,
 };
-FILE *const stderr = &f;
-FILE *volatile __stderr_used = &f;
+FILE *const stderr = &__stderr_FILE;
+FILE *volatile __stderr_used = &__stderr_FILE;

src/stdio/stdin.c

@@ -1,7 +1,9 @@
 #include "stdio_impl.h"
 
+#undef stdin
+
 static unsigned char buf[BUFSIZ+UNGET];
-static FILE f = {
+hidden FILE __stdin_FILE = {
 	.buf = buf+UNGET,
 	.buf_size = sizeof buf-UNGET,
 	.fd = 0,
@@ -11,5 +13,5 @@ static FILE f = {
 	.close = __stdio_close,
 	.lock = -1,
 };
-FILE *const stdin = &f;
-FILE *volatile __stdin_used = &f;
+FILE *const stdin = &__stdin_FILE;
+FILE *volatile __stdin_used = &__stdin_FILE;

src/stdio/stdout.c

@@ -1,7 +1,9 @@
 #include "stdio_impl.h"
 
+#undef stdout
+
 static unsigned char buf[BUFSIZ+UNGET];
-static FILE f = {
+hidden FILE __stdout_FILE = {
 	.buf = buf+UNGET,
 	.buf_size = sizeof buf-UNGET,
 	.fd = 1,
@@ -12,5 +14,5 @@ static FILE f = {
 	.close = __stdio_close,
 	.lock = -1,
 };
-FILE *const stdout = &f;
-FILE *volatile __stdout_used = &f;
+FILE *const stdout = &__stdout_FILE;
+FILE *volatile __stdout_used = &__stdout_FILE;