http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52734
Bug #: 52734 Summary: Incorrect optimization of uClibc sbrk() Classification: Unclassified Product: gcc Version: 4.7.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassig...@gcc.gnu.org ReportedBy: mich...@talamasca.ocis.net Host: i386-pc-linux-gnu Target: i386-pc-linux-gnu Build: i386-pc-linux-gnu Created attachment 27005 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27005 Simplified code analogous to the sbrk problem GCC 4.7.0 miscompiles uClibc 0.9.33 producing an unusable library. The problem is in uClibc's sbrk() implementation -- GCC optimizes away sbrk's attempt to save a global variable before a called function changes it. Basically, that function (brk) is wrongly treated as an __attribute__((pure)) function. The above text is to help anyone searching for the bug. I'm attaching a distilled case showing the same problem. The function ccc() is supposed to return the value of bbb before the second call to aaa(), but when compiled with -Os it returns the value of bbb afterwards. This is on i386-pc-linux-uclibc. GCC 4.6.3 did not have the problem.