[haiku-development] Re: Disabling Strict Aliasing for GCC4 Builds
- From: Stephan Assmus <superstippi@xxxxxx>
- To: haiku-development@xxxxxxxxxxxxx
- Date: Fri, 18 Apr 2008 00:24:20 +0200
Michael Lotz wrote:
> Maybe let me extend the example:
>
> {
> double someDouble = 0.0;
> someDouble += 5.0;
> int variable = *(int *)&someDouble;
> someDouble -= 3.0;
> return 0;
> }
>
> Through strict aliasing rules, it's now like the compiler is allowed to
> assume that something (someDouble in this case) cannot be referenced by
> any pointer as there are no double pointers around. It therefore could
> optimize the variable manipulation of someDouble. It would be allowed to
> put the value into registers and keep it there (not writing the changed
> value back to the memory location) until later on (which is quite
> possible in that example as the variable is used again later in the code
> and would profit from being still in the register). If this was the case,
> then you'd obviously not get the expected value back when reading out
> someDouble by casting it to an int and dereferencing it. You'd only get
> back what is stored at the memory location and not what the real value at
> this point in time (in the register) actually is. I don't really know if
> this would happen with the example above of course, one would have to
> look at the assembler output to check.
Thanks a lot! That explains it perfectly well!
Best regards,
-Stephan
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