On 12 November 2012 22:19, Mickey Iluz <mickey.i...@orcam.com> wrote: > 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.
Hi Mickey. I can reproduce the problem and can't see anything wrong with the code itself. I changed things about to remove the printfs and instead dump the image to the console inside main(). What I see is the whole of the image being correct except for the first column from rows 3 onwards also being set instead of being clear. The fault is cleared with GCC 4.7. I haven't looked further to track down the fix. Could you upgrade to 4.7? -- Michael _______________________________________________ linaro-toolchain mailing list linaro-toolchain@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-toolchain
