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.

1 files changed, 18 insertions, 0 deletions

diff --git a/src/include/stdio.h b/src/include/stdio.h
new file mode 100644
index 00000000..534c6907
--- /dev/null
+++ b/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