Hi,
I've encountered a case where gcc produces a broken program: a branch that
should never be taken is taken, and wrong values are written to memory (and
printed out).
The code is fairly ordinary and small. It can be seen here:
http://pastebin.com/0Hspz8mw
This happens when -funroll-loops flag is used in conjunction with -O2 or -O3.
It doesn't seem to happen when it is used with -O1.
Another few things that influences the program flow from from incorrect to
correct run (gives expected outpus) are:
- Adding/removing printf's inside the inner loop
- Changing the order of the expressions in the "if" clause. i.e. from this:
if ((y < mu) || (y >= H - md) ||
(x < ml) || (x >= W - mr))
to this:
if ((x < ml) || (y >= H - md) ||
(y < mu) || (x >= W - mr))
- Assigning ml inside f() to the same value (3) it's getting from the function
arguments.
All of these shouldn't change how the program behaves but it does.
I compiled this with two different compilers/environments:
1. g++ (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3, running on 3.2.1-42-linaro-lt-mx6
(native compilation on the ARM board)
Compilation command:
g++ -march=armv7-a -mfpu=neon -mfloat-abi=hard -mtune=cortex-a9 -O3
-std=c++0x -funroll-loops -o test_bug_sa_loops_linaro test_bug.cxx
2. arm-fsl-linux-gnueabi-g++ (Freescale MAD -- Linaro 2011.07 -- Built at
2011/08/10 09:20) 4.6.2 20110630 (prerelease)
Running on a freescale LTIB built linux (3.0.15-1359-g1b64ead)
Compilation command:
arm-fsl-linux-gnueabi-g++ -march=armv7-a -mfpu=neon -mfloat-abi=hard
-mtune=cortex-a9 -O3 -std=c++0x -funroll-loops -o test_bug_sa_loops test_bug.cxx
In all the variations I tried it seems that -funroll-loops is critical for this
problem to appear.
I'd be glad to hear some comments on this.
Mickey.
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