Remove the "anything + null => null" optimization

[SingleAccretion]

This optimization is only legal if:

1. "Anything" is a sufficiently small constant itself.
2. We are in a context where we know the address will in fact be used for an indirection.

It is the second point that is problematic - one would like to use MorphAddrContext, but it is not suitable for this purpose, as an unknown context is counted as an indirecting one. Additionally, the value of this optimization is rather low. I am guessing it was meant to support the legacy nullchecks, before GT_NULLCHECK was introduced, and had higher impact then.

So, I propose we just remove the optimization and leave the 5 small regressions across all of SPMI be.

What do the diffs look like?

Just as one would expect:

- mov byte ptr [0000H], 255- ;; bbWeight=1 PerfScore 11.75+ add rcx, -16+ mov byte ptr [rcx], 255

(Note that above is a size improvement)

+ mov esi, ecx+ add rsi, -16
mov rcx, 0xD1FFAB1E ; System.Byte
call CORINFO_HELP_NEWSFAST
; gcrRegs +[rax]
; gcr arg pop 0
- movzx rdx, byte ptr [0000H]+ movzx rdx, byte ptr [rsi]

- mov byte ptr [0000H], 0+ mov ecx, eax+ mov byte ptr [rcx], 0

Fixes#61510

Note I do not plan to do anything here with the GC-ness issue as it is separate.

[ghost]

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Issue Details

---

This optimization is only legal if:

1. "Anything" is a sufficiently small constant itself.
2. We are in a context where we know the address will in fact be used for an indirection.

It is the second point that is problematic - one would like to use MorphAddrContext, but it is not suitable for this purpose, as an unknown context is counted as an indirecting one. Additionally, the value of this optimization is rather low. I am guessing it was meant to support the legacy nullchecks, before GT_NULLCHECK was introduced, and had higher impact then.

So, I propose we just remove the optimization and leave the 5 small regressions across all of SPMI be.

Author:	SingleAccretion
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[SingleAccretion]

@dotnet/jit-contrib

[AndyAyersMS]

Yes, this should go away.

LGTM.

[deeprobin]

LGTM

[AndyAyersMS]

Thank you!