JIT: Enable compaction of all BBJ_ALWAYS blocks

[amanasifkhalid]

Part of #93020. Removes any reference to bbNext in block compaction, and instead considers compacting each block with its jump target regardless of their relative positions in the blocklist. This can churn the flowgraph significantly, but so long as this churn happens before block layout, we can rely on optOptimizeLayout to create a sensible layout regardless of the initial churn. However, since we can run fgUpdateFlowGraph after we've reordered blocks (such as during lowering), I updated fgUpdateFlowGraph's signature to control whether we enable compaction of non-contiguous blocks to avoid changing the block layout.

The logic for updating profile data during compaction looked like it could be simplified, though I'm not sure if my change is overly simplistic; this change seems to have churned fgComputeBlockWeights, hence the large PerfScore diffs. Diffs as a whole are dramatic, though they're inflated largely by libraries_tests.

cc @dotnet/jit-contrib, @AndyAyersMS PTAL. Thanks!

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[AndyAyersMS]

Interesting (and impressive) that this has so much impact.

Can you see if this further improves the case in #4324? RBO was blocked there because of un-compacted blocks.

[AndyAyersMS]

ineheritWeight already takes care of setting the flag.

[amanasifkhalid]

I was running into cases where we'd compact a block with BBF_PROF_WEIGHT set, with a block without the flag, so inheritWeight would unset the flag. I'm guessing we'd want to keep this flag set in such cases, so I added this workaround.

[AndyAyersMS]

Ok. If you remember the cases it might be worth seeing how we end up with a mixture of profiled and unprofiled blocks. Would be nice to systematically reduce how often this happens.

[amanasifkhalid]

I can take another look, but I first noticed this when we would compact a block with profile data with an internal block.

[amanasifkhalid]

Can you see if this further improves the case in #4324? RBO was blocked there because of un-compacted blocks.

Codegen is identical with and without this change; not sure how much of this is driven by weird block layout decisions (without PGO data, ordering conditional blocks' successors is pretty arbitrary), but IG07 suggests there's room to improve?

G_M31901_IG01:  ;; offset=0x0000
       push     rbx
       sub      rsp, 32
       mov      ebx, edx
                                                ;; size=7 bbWeight=1 PerfScore 1.50
G_M31901_IG02:  ;; offset=0x0007
       mov      rcx, gword ptr [rcx+0x08]
       mov      rcx, gword ptr [rcx+0x08]
       test     rcx, rcx
       je       SHORT G_M31901_IG04
                                                ;; size=13 bbWeight=1 PerfScore 5.25
G_M31901_IG03:  ;; offset=0x0014
       mov      r9d, dword ptr [rcx+0x10]
       test     r9d, r9d
       je       SHORT G_M31901_IG06
       mov      rcx, gword ptr [rcx+0x08]
       mov      edx, ebx
       xor      r8d, r8d
       call     [System.Array:IndexOf[int](int[],int,int,int):int]
       mov      ecx, eax
       not      ecx
       shr      ecx, 31
       jmp      SHORT G_M31901_IG07
                                                ;; size=33 bbWeight=0.50 PerfScore 5.88
G_M31901_IG04:  ;; offset=0x0035
       mov      eax, ebx
                                                ;; size=2 bbWeight=0.50 PerfScore 0.12
G_M31901_IG05:  ;; offset=0x0037
       add      rsp, 32
       pop      rbx
       ret
                                                ;; size=6 bbWeight=0.50 PerfScore 0.88
G_M31901_IG06:  ;; offset=0x003D
       xor      ecx, ecx
                                                ;; size=2 bbWeight=0.50 PerfScore 0.12
G_M31901_IG07:  ;; offset=0x003F
       test     ecx, ecx
       je       SHORT G_M31901_IG04
       xor      eax, eax
                                                ;; size=6 bbWeight=0.50 PerfScore 0.75
G_M31901_IG08:  ;; offset=0x0045
       add      rsp, 32
       pop      rbx
       ret
                                                ;; size=6 bbWeight=0.50 PerfScore 0.88

; Total bytes of code 75, prolog size 5, PerfScore 15.38, instruction count 29, allocated bytes for code 75 (MethodHash=031d8362) for method Visitor:Test(int):int:this (FullOpts)
; ============================================================

[AndyAyersMS]

IG07 suggests there's room to improve?

Yeah, IG06/IG07 is a missed case for RBO. No need to retest a value we've just set to zero

[AndyAyersMS]

Regressions:

• [Perf] Windows/x64: 33 Regressions on 6/21/2024 6:31:57 PM #103972

Improvements:

• [Perf] Linux/x64: 7 Improvements on 6/21/2024 6:31:57 PM perf-autofiling-issues#36893
• [Perf] Windows/x64: 2 Improvements on 6/21/2024 6:31:57 PM perf-autofiling-issues#37001 (maybe)