http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47600
Summary: gcc optimizer seems to avoid necessary floating-point
addition
Product: gcc
Version: 4.5.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: acfbuer...@googlemail.com
Created attachment 23237
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23237
assembly and g++ -v output
g++ versions tested: g++-4.5 (Ubuntu/Linaro 4.5.1-7ubuntu2) 4.5.1
g++-4.4 (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5
Problem: I have code like this:
double fakeManipulation(double d) { return d; }
...
dist = <about 100>
const double d = <very small number, 1e-16>;
const double ndist = dist+d; // does not work with optimizer
//const double ndist = fakeManipulation(dist+d); // works with optimizer
if( ndist <= dist )
<avoid endless loop>
The comparison in the if statement is intended exactly like this: check if d is
so small that adding it to dist does not make any difference because of the
limited floating-point precision. I do not know a simpler way to do that which
is equally fast and independent of the actual values of d and dist.
When compiling this code without optimization, the comparison is handled
correctly and the endless loop is avoided.
When compiling with -O or -O2, the comparison is NOT handled correctly and the
program runs into an endless loop. With -O and -O2 and the fakeManipulation
call, it is also handled correctly.
My suspicion is that the optimizer replaces the comparison with (d<=0) which is
not numerically correct.
The full code is rather lengthy and I did not succeed to write a small
demonstration program, so I attached parts of the generated assembler code.
