Re: How do you get the benefit of -fstrict-aliasing?

[Bradley Lucier <lucier at math dot purdue dot edu>]

On Apr 21, 2007, at 6:01 PM, Bradley Lucier wrote:

> So I'm wondering whether certain options have to be included on the  
> command line to get the benefits of -fstrict-aliasing.

I've thought about this question a bit more, so maybe I can make it  
less content-free.

The C code generated by Gambit-C, the Scheme->C compiler, has certain  
somewhat strange characteristics, mainly because it's targeted to a  
simulated virtual machine implemented using C macros.  Experimental  
results show that simply using gcc -O2 or -O3 results in  
significantly suboptimal code; gcse generally makes the code  
noticeably worse, for example.  So we pick and choose our gcc  
optimization options.

The generated code also has significant opportunities to exploit  
alias information---words on the stack cannot alias the car or cdr of  
a pair, the contents of a vector, etc.  The macros implementing the  
virtual machine can be tweaked to make more of that information  
explicit, and I'd like gcc to exploit that information, so I'm  
wondering which optimizations make particular use of aliasing  
information.  Perhaps I've left those off the gcc command line.

Until now, I have used -fno-strict-aliasing in the code for  
correctness reasons (some of the virtual machine macros, related to  
the bignum library and written in a style similar to assembly, have  
aliasing violations).  When I add -fstrict-aliasing when compiling  
user code (which doesn't use the bignum macros), I sometimes get  
significantly slower code. Perhaps there is a lot more register  
pressure as Andrew suggested, but looking at the assembly output  
doesn't seem to justify this assumption (basic blocks are small and  
somewhat independent, it's running on a register-rich machine [64-bit  
PowerPC], etc.).

Brad