Force new label after IG that ends with align instruction

[kunalspathak]

Sometimes, we might end up having 2 jmp instructions as part of same block and we might prefer to add align instruction after the 1st jump but then we would assert that there is no label present. I can update the assert condition to something like below, but I think proper fix would be to set needLabel = true in such cases.

assert(needLabel ||  ((block->bbPrev->bbJumpKind == BBJ_ALWAYS) && (block->bbJumpKind == BBJ_ALWAYS)));

       E97AFFFFFF           jmp      G_M22669_IG12
       E9A0060000           jmp      G_M22669_IG78

Fixes: #61939

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Issue Details

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Sometimes, we might end up having 2 jmp instructions as part of same block and we might prefer to add align instruction after the 1st jump but then we would assert that there is no label present. I can update the assert condition to something like below, but I think proper fix would be to set needLabel = true in such cases.

assert(needLabel ||  ((block->bbPrev->bbJumpKind == BBJ_ALWAYS) && (block->bbJumpKind == BBJ_ALWAYS)));

       E97AFFFFFF           jmp      G_M22669_IG12
       E9A0060000           jmp      G_M22669_IG78

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

[kunalspathak]

@dotnet/jit-contrib @BruceForstall

[BruceForstall]

@kunalspathak Can you explain how we can get back-to-back jmp in the same IG? Wouldn't that make the second one dead code?

[kunalspathak]

That magic happens when we decide to "Conditionally fold" a basic block, but while doing that, do not check if previous block also had an unconditional jump.

Initial flow:

BB16  [0101]  1  0    BB15                  0.90 706    [06A..06B)-> BB22  ( cond ) T0                  keep i nullcheck IBC 
BB17  [0105]  1  0    BB16                  0.90 706    [06A..06B)-> BB22  ( cond ) T0                  i IBC 
BB18  [0106]  1  0    BB17                  0.77 607    [06A..06B)-> BB22  ( cond ) T0                  i IBC 
BB19  [0107]  1  0    BB18                  0.72 569    [06A..06B)-> BB24  ( cond ) T0                  i IBC 
BB20  [0298]  1  0    BB19                  0.72 569    [???..???)-> BB110 (always) T0                  internal IBC 
BB21  [0091]  1  0    BB13                  0.12  98    [04A..04B)-> BB14  (always) T0                  i hascall gcsafe idxlen IBC 
BB22  [0108]  3  0    BB16,BB17,BB18        0.17 137    [06A..06B)                  T0                  i IBC 
BB23  [0110]  1  0    BB22                  0.90 706    [???..???)-> BB110 ( cond ) T0                  keep internal nullcheck IBC 

During conditional folding of jumps, we do this:

Conditional folded at BB23
BB23 becomes a BBJ_ALWAYS to BB110

which transforms the flow to:

BB21  [0091]  1  0    BB13                  0.12  98    [04A..04B)-> BB14  (always) T0                  i hascall gcsafe idxlen IBC 
BB22  [0108]  3  0    BB16,BB17,BB18        0.17 137    [06A..06B)                  T0                  i IBC 
BB23  [0110]  1  0    BB22                  0.90 706    [???..???)-> BB110 (always) T0                  keep internal nullcheck IBC 
BB24  [0015]  3  0    BB19,BB110,BB114      0.71 563    [190..19B)-> BB29  ( cond ) T0                  i nullcheck IBC 
BB25  [0118]  1  0    BB24                  0.32 251    [190..191)-> BB29  ( cond ) T0                  i IBC 

We also optimize and remove BB22 and redirect BB16, BB17 and BB18 to BB110 as part of "Optimizing a jump to an unconditional jump". So BB22 is not left with any predecessor and technically dead.

During codegen, we get this:

BB16  [0101]  1  0    BB15                  0.90 706    [06A..06B)-> BB110 ( cond ) T0                  keep i nullcheck IBC LIR 
BB17  [0105]  1  0    BB16                  0.90 706    [06A..06B)-> BB110 ( cond ) T0                  i IBC LIR 
BB18  [0106]  1  0    BB17                  0.77 607    [06A..06B)-> BB110 ( cond ) T0                  i IBC LIR 
BB19  [0107]  1  0    BB18                  0.72 569    [06A..06B)-> BB24  ( cond ) T0                  i IBC LIR 
BB20  [0298]  1  0    BB19                  0.72 569    [???..???)-> BB110 (always) T0                  internal IBC LIR 
BB165 [0315]  0  0                          0           [???..???)         (throw ) T0                  keep i internal rare label LIR 
BB21  [0091]  1  0    BB13                  0.12  98    [04A..04B)-> BB14  (always) T0                  i label hascall gcsafe idxlen IBC LIR 
BB23  [0110]  0  0                          0.90 706    [???..???)-> BB110 (always) T0                  keep internal nullcheck IBC LIR 

So yes, the second jmp is dead code that stays because of existing shortcoming.

[kunalspathak]

jitstress2-jitstressregs failures are related to #61899
jitstress failures are related to #60330 and #62238