[RFC PATCH] micro-optimize __procfdname

[Alexander Monakov <amonakov@ispras.ru>]

Hello,

I've noticed that internal function __procfdname can be slightly cleaned up by
filling the supplied buffer right-to-left and returning the last filled
position.  The patch below implements what I have in mind, and changes one
call site to demonstrate (I'll be happy to submit a patch that converts all
calls, if overall this change is desirable).

diff --git a/src/internal/procfdname.c b/src/internal/procfdname.c
index 697e0bd..cfb3f90 100644
--- a/src/internal/procfdname.c
+++ b/src/internal/procfdname.c
@@ -1,13 +1,9 @@
-void __procfdname(char *buf, unsigned fd)
+char *__procfdname_impl(char *buf, unsigned fd)
 {
-	unsigned i, j;
-	for (i=0; (buf[i] = "/proc/self/fd/"[i]); i++);
-	if (!fd) {
-		buf[i] = '0';
-		buf[i+1] = 0;
-		return;
-	}
-	for (j=fd; j; j/=10, i++);
-	buf[i] = 0;
-	for (; fd; fd/=10) buf[--i] = '0' + fd%10;
+	*buf = 0;
+	do *--buf = '0' + fd % 10;
+	while (fd /= 10);
+	for (int i = 13; i >= 0; i--)
+		*--buf = "/proc/self/fd/"[i];
+	return buf;
 }
diff --git a/src/internal/procfdname.h b/src/internal/procfdname.h
index e69de29..6d3c6e2 100644
--- a/src/internal/procfdname.h
+++ b/src/internal/procfdname.h
@@ -0,0 +1,9 @@
+#ifndef PROCFDNAME_H
+#define PROCFDNAME_H
+
+char *__procfdname_impl(char *, unsigned);
+
+#define procfdbufsize sizeof "/proc/self/fd/0123456789" + (3 * (sizeof(int)-4))
+#define procfdname(buf, fd) __procfdname_impl(buf + procfdbufsize - 1, fd)
+
+#endif
diff --git a/src/process/fexecve.c b/src/process/fexecve.c
index 6507b42..88e6b9d 100644
--- a/src/process/fexecve.c
+++ b/src/process/fexecve.c
@@ -1,13 +1,11 @@
 #include <unistd.h>
 #include <errno.h>
-
-void __procfdname(char *, unsigned);
+#include "procfdname.h"
 
 int fexecve(int fd, char *const argv[], char *const envp[])
 {
-	char buf[15 + 3*sizeof(int)];
-	__procfdname(buf, fd);
-	execve(buf, argv, envp);
+	char buf[procfdbufsize];
+	execve(procfdname(buf, fd), argv, envp);
 	if (errno == ENOENT) errno = EBADF;
 	return -1;
 }