On Sun, May 13, 2012 at 11:58 PM, H.J. Lu <hjl.to...@gmail.com> wrote:
> On Sun, May 13, 2012 at 12:01 PM, Richard Guenther
> <richard.guent...@gmail.com> wrote:
>> On Sun, May 13, 2012 at 6:32 PM, H.J. Lu <hjl.to...@gmail.com> wrote:
>>> Hi,
>>>
>>> I am trying to optimize memory address for x32. X32 runs in 64-bit mode.
>>> 64-bit address is base + index * scale + offset:
>>> 1. Base is 64bit.
>>> 2. Index is 64bit.
>>> 3. Offset is 8bit or 32bit
>>>
>>> 0x67 address size prefix is used to zero-extend 32bit address to 64bit:
>>> 1. Upper 32bits of base or index can be none-zero.
>>> 2. Index can be negative.
>>>
>>> We can avoid 0x67 prefix if
>>> 1. There is no index
>>> 2. Upper 32bits of base is zero:
>>> a. When base is set via 32bit load op, the upper 32bits are
>>> atomically zero-extended from the lower 32bits
>>> 3. Base + offset is a valid 32-bit address.
>>>
>>> Will x86 backend ever see a memory address with wrap around.
>>> That is to use 0xffffffff + 0x30 to represent 0x2f.
>>
>> Why not? Easily from code like
>>
>> long foo (long a, long b)
>> {
>> return *((long *)(a + b));
>> }
>>
>
> What is the expect run-time behavior when a + b has
> overflow/underflow?
If you make it unsigned long then with the values to quote
you derefernece 0x2f (sorry for using signed arith, and thus
undefinedness on overflow in the example).
Richard.
>
> --
> H.J.
