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Re: Length-Changing Prefixes problem with the x86 Backend


On Wed, Jun 25, 2008 at 11:11 PM, Nadav <nadav256@gmail.com> wrote:

> I came across a problem with the x86 backend of gcc (3.3 - 4.1).  When I
> compile code which access a lookup table of size 65536, gcc generates code
> which uses 16bit opcodes.
>
> The intel architecture optimization guide warns that 16bit opcodes should be
> avoided whenever possible due to Length-Changing Prefixes (LCP).  The
> recommendation is: ""If imm16 is needed, load equivalent imm32 into a
> register and use the word value in the register instead. ""
>
> I would expect gcc to use a 32bits register and mask it, if needed.
> Even when I use 32bits registers and mask them ( & 0xFFFF), gcc detects
> correctly that my variable is effectively only 16bits and it generates
> 16bits code.
> I modified the generated assembly to use 32bit registers and it ran much
> faster.

Please file a bugreport following instructions at
http://gcc.gnu.org/bugs.html. Please also add a runtime test that can
be used to analyze the problem.

Thanks,
Uros.


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