optimization question

[mark maule <mark dot maule at oracle dot com>]

I have a loop which hangs when compiled with -O2, but runs fine when 
compiled with -O1.  Not sure what information is required to get an 
answer, so starting with the full src code.  I have not attempted to 
reduce to a simpler test case yet.

Attachments:

bs_destage.c - full source code
bs_destage.dis.O2 - gdb disassembly of bs_destageLoop()
bs_destage.dis+m.O2 - src annotated version of the above

The function in question is bs_destageSearch().  When I compile 
bs_destage.c with -O2, it seems that the dgHandle condition at line 741 
is being ignored, leading to an infinite loop.  I can see in the 
disassembly that dgHandle is still in the code as a 16-bit value stored 
at 0x32(%rsp), and a running 32-bit copy stored at 0x1c(%rsp).  I can 
also see that the 16 bit version at 0x32(%rsp) is being incremented at 
the end of the loop, but I don't see anywhere in the code where either 
version of dgHandle is being used when determining if the while() at 741 
should be continued.

I'm not very familiar with the optimizations that are done in O2 vs O1, 
or even what happens in these optimizations.

So, I'm wondering if this is a bug, or a subtle valid optimization that 
I don't understand.  Any help would be appreciated.

Note:  changing the declaration of dgHandle to be volitile appears to 
modify the code sufficiently that it looks like the dgHandle check is 
honored (have not tested).

Thanks in advance for any help/advice.

Mark Maule

gcc version:

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.8.3/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man 
--infodir=/usr/share/info 
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-bootstrap 
--enable-shared --enable-threads=posix --enable-checking=release 
--with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions 
--enable-gnu-unique-object --enable-linker-build-id 
--with-linker-hash-style=gnu 
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada,go,lto 
--enable-plugin --enable-initfini-array --disable-libgcj 
--with-isl=/builddir/build/BUILD/gcc-4.8.3-20140911/obj-x86_64-redhat-linux/isl-install 
--with-cloog=/builddir/build/BUILD/gcc-4.8.3-20140911/obj-x86_64-redhat-linux/cloog-install 
--enable-gnu-indirect-function --with-tune=generic --with-arch_32=x86-64 
--build=x86_64-redhat-linux
Thread model: posix
gcc version 4.8.3 20140911 (Red Hat 4.8.3-9) (GCC)

Attachment: bs_destage.c
Description: Text document

Attachment: bs_destage.dis.O2
Description: Text document

Attachment: bs_destage.dis+m.O2
Description: Text document