Created attachment 49050 [details] Test case Hi, I was debugging Compiz test failures that started happening on Debian/Ubuntu s390x in July. They are not directly related to GCC upgrade (old GCC 8 behaves the same way), however I noticed a strange thing that I want to report here. The attached file behaves differently when built with -O0 and -O1: $ g++ test.cpp -g -O1 -lX11 $ xvfb-run ./a.out setting value = 0x1020304 got value = 0xffffffff $ g++ test.cpp -g -O0 -lX11 $ xvfb-run ./a.out setting value = 0x1020304 got value = 0x0 And both these behaviors are wrong! Expected behavior is (as seen on x86_64): $ xvfb-run ./a.out setting value = 0x1020304 got value = 0x1020304 However, this bug is about differing behaviors with -O1 and -O0. The only dependency of this file is Xlib. So far I was not able to make an example that does not depend on it. I wanted to bisect a particular optimization that causes this behavior change, but even with all options from https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-O1 it behaves like -O0. Some more things I noticed: - Moving GetProperty and SetProperty functions away from the namespace makes it behave like -O0. - Adding __attribute__((noinline)) to SetProperty makes it behave like -O0. So the bug may be related to inlining. I am also attaching assembly dumps with -O1 and -O0.
Created attachment 49051 [details] Assembly dump with -O0
Created attachment 49052 [details] Assembly dump with -O1
That is just a user error, you didn't read the XChangeProperty man page. "If the specified format is 16, the property data must be a short array. If the specified format is 32, the property data must be a long array." You are using 32, and I assume you are doing this on 64-bit s390x, i.e. where long is 64-bit, so while it sounds weird, libX11 requires you to put the property into a long or unsigned long variable and pass address of that (and read it through that too). It kind of "works" on x86_64, because it is little-endian, so you just have random bits in the upper 32 bits of the long that the library actually reads and perhaps it casts it to a 32-bit type later on. But s390x is big endian, so if you call it with address of a 32-bit variable, the low 32 bits of that are the 32 bits after your variable and can contain arbitrary random values.
Thanks a lot for the fast response! Indeed, your suggestion works. It is counter-intuitive that you need a 64-bit variable to store a 32-bit value, but I can see the rationale (long is the only standard type that is guaranteed to be 32-bits).