[fpc-devel] Finally fixed that MOVZX/SX optimisation!

J. Gareth Moreton gareth at moreton-family.com
Tue Feb 18 23:04:02 CET 2020

Hi everyone,

So I had some difficulty reproducing a regression with my latest 
optimisation patch, but a couple of days ago I finally reproduced it and 
isolated the cause.  As explained in the topic, my optimisation was 
faulty with the instruction pair "movb $-63,%al; movsbw %al,%ax" and 
other similar constructs - negative constants weren't handled properly.

It seems that not all regressions are caught when you run the test suite 
conventionally, and you have to specify additional options like "-O4", 
which may cause other failures to occur if a test is not expecting any 
kind of intense optimisation, for example.  In response, over here at 
https://bugs.freepascal.org/view.php?id=36687, I've introduced two new 
tests that are copies of the ones that failed, but which explicitly add 
-O4 to the default compiler options.  Besides covering the fault in my 
patch, I can see these tests proving useful later on because I can see 
future peephole optimisations in the compiled assembly language (e.g. 
the following triplet appears... "movw $65280,%ax; cmp $65280,%ax; je 
@lbl"... logical analysis shows that the conditional branch is always 

Long story short, the patch /finally/ works and a full run of the 
regression suite on i386-win32 and x86_64-win64, including the two new 
tests, show no new failures.

Gareth aka. Kit

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.freepascal.org/pipermail/fpc-devel/attachments/20200218/d56c17d9/attachment.html>

More information about the fpc-devel mailing list