Bug 37836

Summary:	Unneeded invocations of memcpy()
Product:	cairo	Reporter:	Steve Snyder <fdt0317.20.swsnyder>
Component:	general	Assignee:	Carl Worth <cworth>
Status:	RESOLVED FIXED	QA Contact:	cairo-bugs mailing list <cairo-bugs>
Severity:	normal
Priority:	medium	CC:	freedesktop
Version:	1.10.3
Hardware:	x86 (IA32)
OS:	All
Whiteboard:
i915 platform:		i915 features:
Attachments:	Skip zero-byte-length call to memcpy()

Description Steve Snyder 2011-06-01 14:37:54 UTC

Created attachment 47444 [details] [review]
Skip zero-byte-length call to memcpy()

In function _cairo_path_fixed_add() (file cairo-path-fixed.c, as pulled from the Thunderbird 5.0 Beta1 source) memcpy() is frequently invoked to copy zero bytes.

My observation with TB 5.0b1 is that in 16% of the calls a byte length of zero is specified.  Further, the code as built by Mozilla using VS2005/VS2008 is generated as functions, not as inline intrinsics.

Not a huge burden, but why incur the cost of these pointless calls to memcpy() if we don't have to?

Comment 1 Behdad Esfahbod 2011-06-01 14:55:39 UTC

I'd patch _cairo_path_buf_add_points() instead.

Comment 2 Steve Snyder 2011-06-01 15:06:09 UTC

(In reply to comment #1)
> I'd patch _cairo_path_buf_add_points() instead.

But the entirety of _cairo_path_buf_add_points() (which is little more than a wrapper around memcpy) is pointless if the num_points variable is zero.

Comment 3 Behdad Esfahbod 2011-06-01 15:15:14 UTC

Sure, but why should the caller bother checking when the callee can do?

Comment 4 Chris Wilson 2011-09-16 08:15:05 UTC

commit 1ce5d4707cf260618bd4d61f39aad4371ffa3336
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Sep 16 16:11:42 2011 +0100

    path: Skip calls to zero-length memcpy
    
    We attempt to copy 0 points onto the array of path points for a
    close-path. This is pointless and an unnecessary function call under
    MSVC at least.
    
    Based on a patch by Steve Snyder, incorporating Behdad's review
    comments.
    
    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=37836
    Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

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