RFR: 8282204: Use lea instructions for arithmetic operations on x86_64 [v11]

[Jie Fu]

On Sat, 12 Mar 2022 02:17:40 GMT, Quan Anh Mai <duke at openjdk.java.net> wrote:

>> src/hotspot/cpu/x86/assembler_x86.cpp line 2393:
>> 
>>> 2391:   InstructionMark im(this);
>>> 2392: #ifdef _LP64
>>> 2393:   emit_int8(0x67); // addr32
>> 
>> By removing the 0x67 prefix, we are now doing 64 bit computation even for 32 bit inputs. Wouldn't this cause partial register stalls (see section 3.5.2.4 end of section).
>
> Partial writes only happen with data size smaller than 32-bit since 32-bit writes are zero-extended to full register width. As a result, there should be no additional stalls here.

> By removing the 0x67 prefix, we are now doing 64 bit computation even for 32 bit inputs. Wouldn't this cause partial register stalls (see section 3.5.2.4 end of section).

I agree with @merykitty .
The manual mentions

Note that in Intel 64 architecture, an update to the lower 32 bits of a 64 bit integer register is architecturally defined to zero extend the upper 32 bits. While this action may be logically viewed as a 32 bit
update, it is really a 64 bit update (and therefore does not cause a partial stall).


TBH, I didn't know this opt rule before.
Thanks @sviswa7 for sharing this.

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PR: https://git.openjdk.java.net/jdk/pull/7560