This seems to be a bug in gcc-3.3.1. -fstrict-aliasing is enabled by -O2 or higher optimization in gcc 3.3.1.
Now that I think of it, they might be talking about an optimization called register aliasing, where they are taking the structure and mapping it to a CPU register for some optimization, and what we are doing is to store a different structure in there that will not fit in a register. A Node will fit in a register (it is only an enum) but the TriggerData structure will not, meaning the code has to spill the register to memory, then access the full structure, or something like that.
Did you mean register renaming? If so, it is only turned on by -O3. But I can see that strict aliasing does help the compiler make the right guess as to which registers to use for what, even without register renaming.
http://gcc.gnu.org/onlinedocs/gcc-3.3.1/gcc/Optimize-Options.html#Optimize%20Options says:
|-O|
|-O1| Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.
With |-O|, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.
|-O| turns on the following optimization flags:
-fdefer-pop -fmerge-constants -fthread-jumps -floop-optimize -fcrossjumping -fif-conversion -fif-conversion2 -fdelayed-branch -fguess-branch-probability -fcprop-registers
|-O| also turns on |-fomit-frame-pointer| on machines where doing so does not interfere with debugging.
|-O2| Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. The compiler does not perform loop unrolling or function inlining when you specify |-O2|. As compared to |-O|, this option increases both compilation time and the performance of the generated code.
|-O2| turns on all optimization flags specified by |-O|. It also turns on the following optimization flags:
-fforce-mem -foptimize-sibling-calls -fstrength-reduce -fcse-follow-jumps -fcse-skip-blocks -frerun-cse-after-loop -frerun-loop-opt -fgcse -fgcse-lm -fgcse-sm -fdelete-null-pointer-checks -fexpensive-optimizations -fregmove -fschedule-insns -fschedule-insns2 -fsched-interblock -fsched-spec -fcaller-saves -fpeephole2 -freorder-blocks -freorder-functions -fstrict-aliasing -falign-functions -falign-jumps -falign-loops -falign-labels
Please note the warning under |-fgcse| about invoking |-O2| on programs that use computed gotos.
|-O3| Optimize yet more. |-O3| turns on all optimizations specified by |-O2| and also turns on the |-finline-functions| and |-frename-registers| options.
In the Linux kernel, you can see this in include/linux/tcp.h:
/*
* The union cast uses a gcc extension to avoid aliasing problems
* (union is compatible to any of its members)
* This means this part of the code is -fstrict-aliasing safe now.
*/
union tcp_word_hdr {
struct tcphdr hdr;
__u32 words[5];
};#define tcp_flag_word(tp) ( ((union tcp_word_hdr *)(tp))->words [3])
Maybe this gives us a clue.
cheers
andrew
---------------------------(end of broadcast)--------------------------- TIP 2: you can get off all lists at once with the unregister command (send "unregister YourEmailAddressHere" to [EMAIL PROTECTED])
