http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48784
Summary: #pragma pack(1) + -fstrict-volatile-bitfields = bad codegen Product: gcc Version: 4.7.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization AssignedTo: unassig...@gcc.gnu.org ReportedBy: arthur.j.odw...@gmail.com Created attachment 24109 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24109 Output of "ajo-gcc -fstrict-volatile-bitfields bug868575724-reduced.c -v" #pragma pack(1) is incompatible with -fstrict-volatile-bitfields, causing silently wrong codegen if you mix the two options. It feels like the compiler really ought to warn the user that bad code will be generated, even if it *is* ultimately the user's fault. ...But is it even the user's fault, in this case? cat >test.c <<EOF #include <stdio.h> #pragma pack(1) volatile struct S0 { signed a : 7; unsigned b : 28; /* b can't be fetched with an aligned 32-bit access, */ /* but it certainly can be fetched with an unaligned access */ } g = {0,-1}; int main() { printf("%x\n", (unsigned int)g.b); return 0; } EOF gcc test.c ; ./a.out // prints "fffffff" gcc -fstrict-volatile-bitfields test.c ; ./a.out // prints "1ffffff" Without -fstrict-volatile-bitfields, the correct 28-bit number "fffffff" is printed. With -fstrict-volatile-bitfields, the incorrect 25-bit number "1ffffff" is printed. Bug 43341 is a similar gray-area bug/not-a-bug involving #pragma pack. This test case is reduced from the output of Csmith (http://embed.cs.utah.edu/csmith/), using the following command line: csmith --bitfields --packed-struct -s 868575724 > test868575724.c