[Bug c/39330] Bad program bahavior with -O2 optimization in plain C

[ebotcazou at gcc dot gnu dot org]

--- Comment #4 from ebotcazou at gcc dot gnu dot org  2009-03-01 08:07 
---
 There is a bug in the gcc compiler for the C code that changes the behavior of
 a simple program with -O2 optimizations, but not with -O1 or -O0.

-O2 enables -fstrict-aliasing so the code must be written in ISO C as far as
aliasing is concerned, otherwise the behavior is undefined.

Your code is not written in ISO C:

#define VL(X) (*((uint32*)(X)))

is a direct violation of ISO C.  See the -fstrict-aliasing entry in the GCC
manual or the ISO standard.


-- 

ebotcazou at gcc dot gnu dot org changed:

   What|Removed |Added

 CC||ebotcazou at gcc dot gnu dot
   ||org
 Status|UNCONFIRMED |RESOLVED
 Resolution||INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39330

[Bug c/39330] Bad program bahavior with -O2 optimization in plain C

[adeymo at dc dot uba dot ar]

--- Comment #1 from adeymo at dc dot uba dot ar  2009-03-01 05:05 ---
Created an attachment (id=17379)
 -- (http://gcc.gnu.org/bugzilla/attachment.cgi?id=17379action=view)
Testcase with debug information and explanation

Compile with:
gcc -save-temps -O2 -Wall -Werror xqsort.c -o xqsort




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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39330

[Bug c/39330] Bad program bahavior with -O2 optimization in plain C

[adeymo at dc dot uba dot ar]

--- Comment #2 from adeymo at dc dot uba dot ar  2009-03-01 05:06 ---
Created an attachment (id=17380)
 -- (http://gcc.gnu.org/bugzilla/attachment.cgi?id=17380action=view)
Testcase smaller and simpler


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39330

[Bug c/39330] Bad program bahavior with -O2 optimization in plain C

[adeymo at dc dot uba dot ar]

--- Comment #3 from adeymo at dc dot uba dot ar  2009-03-01 05:08 ---
(From update of attachment 17380)
Compile with:
gcc -save-temps -O2 -Wall -Werror xqsort-small.c -o xqsort-small

Run with:
./xqsort-small  echo ok

If compiled with -O1 , the echo command must be executed.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39330