[wasm] Add intrinsics for clz and ctz instructions

[radekdoulik]

Add intrinsics for leading and trailing zero count for wasm.

Introduce new internal class WasmBase for non-SIMD wasm intrinsics. These are always used in AOT builds, unlike SIMD intrinsics, which are disabled by default.

[radekdoulik]

Example of the relevant code change:

$ wa-diff -d -f SpanHelper.*Index bin-simd\Release\AppBundle\dotnet.wasm bin-simd-base\Release\AppBundle\dotnet.wasm
(func corlib_System_SpanHelpers_IndexOfValueType_byte__byte_int(param i32, i32, i32, i32) (result i32))
...
       local.get $9
       i8x16.bitmask    [SIMD]
-      local.tee $6
+      i32.ctz
-      i32.eqz
-      if
-       local.get $4
-       i32.const 32
-       i32.add
-       return
-
-      i32.const 2131592
-      i32.load align:2
-      local.get $6
-      i32.const 0
-      local.get $6
-      i32.sub
-      i32.and
-      i32.const 125613361
-      i32.mul
-      i32.const 27
-      i32.shr.u
-      i32.add
-      local.tee $6
-      i32.eqz
-      br.if
-      local.get $6
-      i32.load8.u
       local.get $4
       i32.add
...

[radekdoulik]

/azp run runtime-wasm-perf

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[radekdoulik]

I measured the performance in SpanHelper, where it saves few us as it is used only once. Let see if microbenchmarks can show other areas where it might help more.

The size saving is nice, I see cca 1k reduction in bench sample's dotnet.wasm in default aot Release configuration and cca 3k for SIMD Release configuration.

[tannergooding]

Why not just handle BitOperations.LeadingZeroCount and BitOperations.TrailingZeroCount directly?

[radekdoulik]

I wanted to stay close to the other platforms implementation.

[tannergooding]

The others exist largely due to historical reasons and because on x86 they aren't part of the "baseline" instruction set. There will ultimately be better codegen and less work for the compiler if you recognize the BitOperations APIs directly. This is because the compiler won't have to do inlining, dead code elimination for n platforms, etc.

I'm planning on doing the same for RyuJIT and recognizing BitOperations directly for the same reasons.

[radekdoulik]

What do you plan to do with the Log2 and RoundUpToPowerOf2 methods? Implement them as direct intrinsics too?

[tannergooding]

For Log2 yes. For RoundUpToPowerOf2 that's TBD, but potentially.

tzcnt and lzcnt are the most hot methods and the most impactful in terms of "compiler work" required to inline and optimize down to a single instruction today.

[vargaz]

An alternative would be to move the if (cfg->opt check to mono_emit_simd_intrinsics ()).

[radekdoulik]

The failing tests are unrelated and failing in main and in other PRs too.

[radekdoulik]

I will address the feedback in following PR.