Subject: Re: Optimizing away "ReadFile" calls when Make calls stat()
From: Warren Young <[email-masked]>
To: Cygwin-L <[email-masked]>
Date: Tue, 13 Feb 2001 16:28:04 -0700

[email-masked] wrote:
> 
> As I've noted separately, reading tens of thousands of files even once
> incurs a significant performance penalty.  The change I've proposed
> can eliminate reading them at all.

Even stat() under Linux does at least one disk read.  You can't
completely optimize away disk I/O for stat().

The main culprit is that this is one of many places where Unix doesn't
map onto Win32 well at all.  The VC++ RTL doesn't use ReadFile() to
implement _stat() at all.  When it checks for things like stat.st_mode
== S_IEXEC, it simply checks the filename extension for .exe, .com or
.bat.  Cygwin can't do that -- it must look at the file's magic bytes to
see if it's an executable, or a #! style script.  

stat() on Unixen doesn't do either of these things; all the info stat()
reports is in the inode.  The info in a prototypical Unix inode is
scattered in many different places in Win32, which makes the emulator
for a call like stat() slow.

Maybe a better optimization strategy would be to patch GNU Make. 
Wherever it does a stat() to find the modification time, do something
like this:

        struct stat st;
#ifdef CYGWIN
        WIN32_FIND_DATA findinfo;
        HANDLE h = FindFirstFile(filename, &findinfo);
        st.st_mtime = findinfo.ftLastWriteTime;
        FindClose(h);
#else
        stat(filename, &st);
#endif

(ftLastWriteTime will probably need translation into a time_t, but
that's a small matter.)
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