Clear some stale GTF_ORDER_SIDEEFF flags

[dhartglassMSFT]

Clear stale GTF_ORDER_SIDEEFF flags to enable more if-conversion and optimize bools.

Motivating example was o.GetType() == typeof(int) || o.GetType() == typeof(uint) || o.GetType() == typeof(long) - we couldn't combine into OR because leftover side effect bit in one of the blocks.

If an op doesn't itself support side effects, and none of node's children have the bit, then the node may have the bit cleared.

Diffs show more optimize bools+if conversion taking place.
One of the bad diffs was block layout changing and causing LSRA to spill slightly different.
Another bad diff exposed an issue where edge-likelihoods weren't recalculated properly after optimize bools, and caused perfscore to balloon. I'm working on a fix for that seperately.

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[Copilot]

Pull request overview

This PR adjusts JIT tree side-effect flag maintenance by attempting to clear stale GTF_ORDER_SIDEEFF bits during Compiler::gtUpdateStmtSideEffects, to unblock downstream optimizations that are inhibited by incorrectly-propagated ordering constraints.

Changes:

• Add logic in gtUpdateStmtSideEffects to clear GTF_ORDER_SIDEEFF on nodes that don’t support ordering side effects when none of their operands still carry the flag.

[dhartglassMSFT]

seems reasonable

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

[dhartglassMSFT]

link to diffs https://dev.azure.com/dnceng-public/public/_build/results?buildId=1486826&view=ms.vss-build-web.run-extensions-tab

Example (more ccmp in a method similar to the motivating example)

-4 (-7.14%) : AggregateSymbol:IsRefType():bool:this (FullOpts)

@@ -22,30 +22,27 @@ G_M9025_IG02:        ; bbWeight=1, gcrefRegs=0001 {x0}, byrefRegs=0000 {}, byref
             ; gcrRegs +[x0]
             ldr     w1, [x0, #0x90]
             cmp     w1, #1
-            beq     G_M9025_IG04
+            bne     G_M9025_IG05
                         ;; size=12 bbWeight=1 PerfScore 4.50
-G_M9025_IG03:        ; bbWeight=0.50, gcrefRegs=0000 {}, byrefRegs=0000 {}, byref, isz
+G_M9025_IG03:        ; bbWeight=0.50, gcrefRegs=0000 {}, byrefRegs=0000 {}, byref
             ; gcrRegs -[x0]
-            cmp     w1, #3
-            bne     G_M9025_IG06
-                        ;; size=8 bbWeight=0.50 PerfScore 0.75
-G_M9025_IG04:        ; bbWeight=0.50, gcrefRegs=0000 {}, byrefRegs=0000 {}, byref
             mov     w0, #1
                         ;; size=4 bbWeight=0.50 PerfScore 0.25
-G_M9025_IG05:        ; bbWeight=0.50, epilog, nogc, extend
+G_M9025_IG04:        ; bbWeight=0.50, epilog, nogc, extend
             ldp     fp, lr, [sp], #0x10
             ret     lr
                         ;; size=8 bbWeight=0.50 PerfScore 1.00
-G_M9025_IG06:        ; bbWeight=0.50, gcVars=0000000000000000 {}, gcrefRegs=0000 {}, byrefRegs=0000 {}, gcvars, byref
-            cmp     w1, #2
+G_M9025_IG05:        ; bbWeight=0.50, gcVars=0000000000000000 {}, gcrefRegs=0000 {}, byrefRegs=0000 {}, gcvars, byref
+            cmp     w1, #3
+            ccmp    w1, #2, z, ne
             cset    x0, eq
-                        ;; size=8 bbWeight=0.50 PerfScore 0.50
-G_M9025_IG07:        ; bbWeight=0.50, epilog, nogc, extend
+                        ;; size=12 bbWeight=0.50 PerfScore 0.75
+G_M9025_IG06:        ; bbWeight=0.50, epilog, nogc, extend
             ldp     fp, lr, [sp], #0x10
             ret     lr
                         ;; size=8 bbWeight=0.50 PerfScore 1.00
 
-; Total bytes of code 56, prolog size 8, PerfScore 9.50, instruction count 14, allocated bytes for code 56 (MethodHash=4337dcbe) for method Microsoft.CSharp.RuntimeBinder.Semantics.AggregateSymbol:IsRefType():bool:this (FullOpts)
+; Total bytes of code 52, prolog size 8, PerfScore 9.00, instruction count 13, allocated bytes for code 52 (MethodHash=4337dcbe) for method Microsoft.CSharp.RuntimeBinder.Semantics.AggregateSymbol:IsRefType():bool:this (FullOpts)
 ; ============================================================

[AndyAyersMS]

I assume we end up in this state because we fail to clear the flag when we change the oper?

Should we add an IR check that we never see GTF_ORDER_SIDEEFF on a node where it isn't expected?

[jakobbotsch]

Suggested change

	if (!tree->OperSupportsOrderingSideEffect())
	if (((tree->gtFlags & GTF_ORDER_SIDEEFF) != 0) && !tree->OperSupportsOrderingSideEffect())

For TP

[jakobbotsch]

I assume we end up in this state because we fail to clear the flag when we change the oper?

Should we add an IR check that we never see GTF_ORDER_SIDEEFF on a node where it isn't expected?

I agree it would be nice to make fgDebugCheckFlags check this flag more thoroughly. There will likely be some fallout if you do that -- for one you will probably need to change gtUpdateNodeOperSideEffects to remove this flag. Morph uses that to do a similar update of flags as gtUpdateStmtSideEffects inline during its walk.

[dhartglassMSFT]

I assume we end up in this state because we fail to clear the flag when we change the oper?
Should we add an IR check that we never see GTF_ORDER_SIDEEFF on a node where it isn't expected?

I agree it would be nice to make fgDebugCheckFlags check this flag more thoroughly. There will likely be some fallout if you do that -- for one you will probably need to change gtUpdateNodeOperSideEffects to remove this flag. Morph uses that to do a similar update of flags as gtUpdateStmtSideEffects inline during its walk.

@jakobbotsch + @AndyAyersMS sequence of steps here was, null check gets removed on an IND so it + the parent EQ is marked SIDEEFFECT:

 [000019] -----O-----   JTRUE
 [000018] J----O-N---     EQ
 [000017] #----O-----        IND    long
 [000011] -----+-----           LCL_VAR ref V00
 [000013] H----+-----        CNS_INT(h) ...

IND gets CSE'd with a local, but EQ retains the bit (the gtUpdateStmtSideEffects method im changing here is called immediately after the CSE)

 [000018] J----O-N---   EQ         int 
 [000037] -----------      LCL_VAR long V02 cse0 
 [000013] H----+-----      CNS_INT(h) ...

[dhartglassMSFT]

Opened #130150 to track the bad diff with the ballooned perfscore