Help ensure we get the smallest encoding for commutative instructions

[tannergooding]

The VEX 2-byte encoding allows REX.R to be on or off while it requires that REX.B be off. This means that op1 can encode [XMM0, XMM15] while op2 can only encode `[XMM0, XMM7]. Thus, when we have a commutative instruction we want to check for this scenario and swap the operands. If both are not "extended" then we'll get the 2-byte encoding regardless, while if both are extended we'll require the 3-byte encoding.

This doesn't handle some of the comparison cases that also qualify but which require swapping the comparison that is being done.

- vpand    xmm3, xmm3, xmm8
-                        ;; size=5
+ vpand    xmm3, xmm8, xmm3
+                        ;; size=4

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[Copilot]

Pull request overview

This PR introduces a new “commutative” instruction flag for xarch SIMD/AVX instructions and uses it in the xarch emitter to opportunistically swap op1/op2 for commutative 3-operand SIMD instructions, improving the chances of using a smaller (2-byte) VEX prefix when only one source register is “extended”.

Changes:

• Add INS_Flags_IsCommutative to insFlags and mark a set of commutative SIMD instructions with it.
• Add emitter::IsAvxCommutative and use it in emitIns_SIMD_R_R_R to swap operands when op2 is extended but op1 is not.
• Wire the new helper into the emitter’s xarch interface.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/coreclr/jit/instrsxarch.h	Marks specific VEX/EVEX SIMD instructions as commutative via INS_Flags_IsCommutative.
src/coreclr/jit/instr.h	Introduces the new INS_Flags_IsCommutative bit in the xarch insFlags enum.
src/coreclr/jit/emitxarch.h	Declares the new emitter::IsAvxCommutative helper.
src/coreclr/jit/emitxarch.cpp	Implements IsAvxCommutative and adds operand-swapping logic in emitIns_SIMD_R_R_R.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.