RFR: 8358333: Use VEX2 prefix in Assembler::psllq

[Emanuel Peter]

On Mon, 2 Jun 2025 16:02:30 GMT, Yudi Zheng <yzheng at openjdk.org> wrote:

>> While porting the commit https://github.com/openjdk/jdk/commit/0df8c9684b8782ef830e2bd425217864c3f51784 to Graal, I noticed that the Assembler::psllq instruction is using the VEX3 prefix. This results in the instruction being unrecognizable by my outdated version of hsdis. Currently, HotSpot generates the following bytes for vpsllq xmm7, xmm7, 0x34
>> https://github.com/openjdk/jdk/blob/0df8c9684b8782ef830e2bd425217864c3f51784/src/hotspot/cpu/x86/stubGenerator_x86_64_cbrt.cpp#L255
>> 
>> 
>> c4 e1 c1 73 f7 34
>> 
>> 
>> By setting the rex_w to WIG, the emitted bytes are:
>> 
>> 
>> c5 c1 73 f7 34
>
> @jatin-bhateja could you please review this trivial PR? Thanks!

@mur47x111 Testing was only run for tier1-3. It would be better if there was additional stress testing as well.

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PR Comment: https://git.openjdk.org/jdk/pull/25593#issuecomment-2935143384