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Re: optimization/5159: ptr[i] = ptr[++i]; // works differently with malloc() and -O
- From: neil at gcc dot gnu dot org
- To: gcc-bugs at gcc dot gnu dot org, gcc-prs at gcc dot gnu dot org, nobody at gcc dot gnu dot org, robc at free dot fr
- Date: 18 Dec 2001 23:22:36 -0000
- Subject: Re: optimization/5159: ptr[i] = ptr[++i]; // works differently with malloc() and -O
- Reply-to: neil at gcc dot gnu dot org, gcc-bugs at gcc dot gnu dot org, gcc-prs at gcc dot gnu dot org, nobody at gcc dot gnu dot org, robc at free dot fr, gcc-gnats at gcc dot gnu dot org
Synopsis: ptr[i] = ptr[++i]; // works differently with malloc() and -O
State-Changed-From-To: open->closed
State-Changed-By: neil
State-Changed-When: Tue Dec 18 15:22:36 2001
State-Changed-Why:
Not a bug - you should read up on sequence points. Basically, multiple reads of a variable (i) modified between sequence points is undefined. Your code falls into this category. So,
ptr[i] = ptr[++i];
is fundamentally always meaningless.
http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=5159