ARM64: Left over dead code for special throw blocks

[kunalspathak]

While debugging something on ARM64, I noticed that we end up leaving some of the dead code in place.

private static uint DivTest(uint xr)
{
 return xr / 4;
}

Generates the following where IG04 block is dead:

G_M26531_IG01:
 A9BF7BFD stp fp, lr, [sp,#-16]!
 910003FD mov fp, sp
 ;; bbWeight=1 PerfScore 1.50
G_M26531_IG02:
 53027C00 lsr w0, w0, #2
 ;; bbWeight=1 PerfScore 1.00
G_M26531_IG03:
 A8C17BFD ldp fp, lr, [sp],#16
 D65F03C0 ret lr
 ;; bbWeight=1 PerfScore 2.00
G_M26531_IG04:
 97FF67A9 bl CORINFO_HELP_THROWDIVZERO
 D43E0000 bkpt

For int parameter, we have 2 blocks that are dead:

G_M26531_IG01:
 A9BF7BFD stp fp, lr, [sp,#-16]!
 910003FD mov fp, sp
 ;; bbWeight=1 PerfScore 1.50
G_M26531_IG02:
 131F7C01 asr w1, w0, #31
 12000421 and w1, w1, #3
 0B000020 add w0, w1, w0
 13027C00 asr w0, w0, #2
 ;; bbWeight=1 PerfScore 3.00
G_M26531_IG03:
 A8C17BFD ldp fp, lr, [sp],#16
 D65F03C0 ret lr
 ;; bbWeight=1 PerfScore 2.00
G_M26531_IG04:
 97FF67A2 bl CORINFO_HELP_OVERFLOW
 ;; bbWeight=0 PerfScore 0.00
G_M26531_IG05:
 97FF67A5 bl CORINFO_HELP_THROWDIVZERO
 D43E0000 bkpt

We don't produce dead BB if I change from division to bit-shift like xr >> 2. I was assuming there be some dead code elimination phase, but looking at dumps I didn't see anything like that happening.

category:cq
theme:dead-code
skill-level:intermediate
cost:medium

[Dotnet-GitSync-Bot]

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[kunalspathak]

@dotnet/jit-contrib

[kunalspathak]

Looks like we add those throwBlocks inside morph for ARM64 and then don't remove internal throw blocks inside flowgraph. We can perhaps eliminate 1 or both those checks in certain situation where e.g. divisor is a constant power of 2 (no need of overflow check) or if it is non-zero constant (no need of divide by zero check).

[AndyAyersMS]

Marking as future.