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Date: Sun, 18 Jan 2026 12:00:40 -0800 (PST)
To: "cygwin@cygwin.com" <cygwin@cygwin.com>
Message-ID: <226987210.591128.1768766440723@connect.xfinity.com>
Subject: Log2 seems inefficient
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From: KENNON J CONRAD via Cygwin <cygwin@cygwin.com>
Reply-To: KENNON J CONRAD <kennonconrad@comcast.net>
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Hello,
 
   I am not an experienced cygwin user but have been using it to compile code on Windows (gcc), so I may be leaving out some key details.  My issue is that for every log2 call in my code there is a divide that follows a log library call in the assembly code.  It appears to be related to these lines in math.h:
 
#if !defined(__cplusplus)
#define log2(x) (log (x) / _M_LN2)
#endif
 
My questions are:  If log(x) is used, wouldn't it be faster to multiply by constant (1/LN2) instead of divide by constant (LN2)?  And wouldn't it be even more efficient to compute log2 directly instead of computing the natural log and then converting it to log2?
 
I realize this is not a "bug", it just doesn't seem very optimal from an speed standpoint.
 
Best Regards,
 
Kennon Conrad
 
 

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