[9] RFR(S): 8075324: Costs of memory operands in aarch64.ad are inconsistent (original) (raw)

Andrew Haley aph at redhat.com
Tue Mar 17 15:37:51 UTC 2015

On 03/17/2015 03:15 PM, Andrew Dinn wrote:

If you take look at the static methods named loadStore defined in the source block you can see how this instruction will actually be generated. PrintAssembly will reveal that it gets translated as follows

add R8, R10, #16 ldrsbw R0, [R8, R1 sxtw]

Note that this is faster than ...

 add     R8, R10, R1 sxtw
 ldrsbw  R0, [R8, #16]

... because sign-extending adds are "free" in address arithmetic but not in regular arithmetic. Why? Answers on a postcard...

Therefore, the costs of these two forms shouldn't be equal. I think we have this correct at present.

Andrew.

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