[Bug middle-end/39421] New: Wrong code for optimizition on 64-bit scalar integers

[ktietz at gcc dot gnu dot org]

All of these problems are adisappearing when the code is compiled without the
-O2 flag. 

I have tried to reproduce the problems using simple code fragments, but I have
only succeeded in doing this for 2 of these problems (t1 and t3 in the attached
testcase). I cannot reproduce the other problem (t2 in the following code).
Running the program (with -O2) produced the following: 

$ ~/a.exe 
w is 8000000000000000 
, V is 8000000000000000 
t1 Wrong! 
v != w 
t2 Correct! 
t3: w = 8000000000000000 
t3 Wrong! 

In our actual code corresponding to t2, we do the same comparison of  
if ((uword) w > (uword) MX_S_W)) 

and the comparison fails when w was originally negative.  

For t1 and t3, I can see why some optimisation might reason away the actual
tests, if it does not consider overflow of the signed word. Curiously, removing
the printf before if (w <= 0) ... line in t3 cause the  
if (w <= 0) test to fail -- I don't see why this happens. 

(in our original code, we don't have the printf, but w <= 0 does succeed, it is
the w == MN_S_W test that fails as in t3).


-- 
           Summary: Wrong code for optimizition on 64-bit scalar integers
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ktietz at gcc dot gnu dot org
GCC target triplet: x86_64-*-*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39421