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Special case Mono in SpanHelpers.Fill/CleanWithoutReference #99059

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Special case Mono in SpanHelpers.Fill/CleanWithoutReference #99059

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6b9c790
Special case Mono
EgorBo Feb 28, 2024
8d78573
Update SpanHelpers.ByteMemOps.cs
EgorBo Feb 28, 2024
b1a1bf2
Update SpanHelpers.ByteMemOps.cs
EgorBo Feb 28, 2024
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9 changes: 9 additions & 0 deletions src/libraries/System.Private.CoreLib/src/System/Span.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -300,6 +300,15 @@ public unsafe void Clear()
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public unsafe void Fill(T value)
{
// Mono works faster with Unsafe.InitBlockUnaligned
#if MONO
if (sizeof(T) == 1)
{
Unsafe.InitBlockUnaligned(ref Unsafe.As<T, byte>(ref _reference), *(byte*)&value, (uint)_length);
return;
}
#endif
Comment on lines +304 to +310
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Is it worth doing this over updating Mono to recognize/special-case the same scenarios as dotnet/runtime?

At the very least, should we ensure an issue tracking that same support is added so this #if MONO can be removed long term?

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Not sure whether Mono folks want to add more intrinsics in Mini and Interpreter considering this short path just works as is, so leaving that decision up to them, cc @BrzVlad

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I'm not a big fan of having MONO ifdefs in SPC. Also I think the regression is limited to interpreter, so disabling for all mono might be uncalled for. Also this should be easy to intrinsify, I'll take a look.

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@matouskozak matouskozak Feb 29, 2024 •
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I'm not a big fan of having MONO ifdefs in SPC. Also I think the regression is limited to interpreter, so disabling for all mono might be uncalled for. Also this should be easy to intrinsify, I'll take a look.

The regression can be seen for mini (JIT) as well (https://pvscmdupload.blob.core.windows.net/reports/allTestHistory/refs/heads/main_x64_ubuntu%2022.04_LLVM=false_MonoAOT=false_MonoInterpreter=false_RunKind=micro_mono/System.Memory.Span(Char).Clear(Size:%2033).html)
image

Most likely for AOT as well since, the microbenchmark is written using generics, however, our AOT perf lines are currently broken.

Having Mono ifdefs in managed code doesn't seem to be a good long-term solution. If we go with this solution and the solution proves to remove the regression, I think we should add a tracking issue for adding these intrinsics to Mono. What do you think about this @fanyang-mono?

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I took a look a quick look at the original PR (#98623). It seems to me that to intrinsify SpanHelpers.Memmove, the code path for intrinsifying Buffer.Memmove was used with some tweak. Mono intrinsify Buffer.Memmove as well. See the code below.

runtime/src/mono/mono/mini/intrinsics.c

Lines 288 to 310 in fd48b6f

if (in_corlib && !strcmp (m_class_get_name (cmethod->klass), "Buffer")) {
if (!strcmp (cmethod->name, "Memmove") && fsig->param_count == 3 && m_type_is_byref (fsig->params [0]) && m_type_is_byref (fsig->params [1]) && !cmethod->is_inflated) {
MonoBasicBlock *end_bb;
NEW_BBLOCK (cfg, end_bb);
// do nothing if len == 0 (even if src or dst are nulls)
MONO_EMIT_NEW_BIALU_IMM (cfg, OP_COMPARE_IMM, -1, args [2]->dreg, 0);
MONO_EMIT_NEW_BRANCH_BLOCK (cfg, OP_IBEQ, end_bb);
// throw NRE if src or dst are nulls
MONO_EMIT_NEW_BIALU_IMM (cfg, OP_COMPARE_IMM, -1, args [0]->dreg, 0);
MONO_EMIT_NEW_COND_EXC (cfg, EQ, "NullReferenceException");
MONO_EMIT_NEW_BIALU_IMM (cfg, OP_COMPARE_IMM, -1, args [1]->dreg, 0);
MONO_EMIT_NEW_COND_EXC (cfg, EQ, "NullReferenceException");
MONO_INST_NEW (cfg, ins, OP_MEMMOVE);
ins->sreg1 = args [0]->dreg; // i1* dst
ins->sreg2 = args [1]->dreg; // i1* src
ins->sreg3 = args [2]->dreg; // i32/i64 len
MONO_ADD_INS (cfg->cbb, ins);
MONO_START_BB (cfg, end_bb);
}
}

I think the proper fix should be updating the above code as CoreCLR did.

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Created a github issue for it #99161


SpanHelpers.Fill(ref _reference, (uint)_length, value);
}

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24 changes: 12 additions & 12 deletions src/libraries/System.Private.CoreLib/src/System/SpanHelpers.ByteMemOps.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -254,6 +254,13 @@ public static unsafe void ClearWithoutReferences(ref byte dest, nuint len)
if (len == 0)
return;

// Mono works faster with Unsafe.InitBlockUnaligned
#if MONO
if (len > ZeroMemoryNativeThreshold)
goto PInvoke;
Unsafe.InitBlockUnaligned(ref dest, 0, (uint)len);
return;
#else
ref byte destEnd = ref Unsafe.Add(ref dest, len);

if (len <= 16)
Expand Down Expand Up @@ -427,6 +434,7 @@ public static unsafe void ClearWithoutReferences(ref byte dest, nuint len)
Unsafe.WriteUnaligned<int>(ref Unsafe.Add(ref destEnd, -4), 0);
#endif
return;
#endif // MONO

PInvoke:
// Implicit nullchecks
Expand Down Expand Up @@ -498,24 +506,16 @@ internal static void Fill(ref byte dest, byte value, nuint len)
nuint stopLoopAtOffset = len & ~(nuint)7;
do
{
Unsafe.Add(ref dest, (nint)i + 0) = value;
Unsafe.Add(ref dest, (nint)i + 1) = value;
Unsafe.Add(ref dest, (nint)i + 2) = value;
Unsafe.Add(ref dest, (nint)i + 3) = value;
Unsafe.Add(ref dest, (nint)i + 4) = value;
Unsafe.Add(ref dest, (nint)i + 5) = value;
Unsafe.Add(ref dest, (nint)i + 6) = value;
Unsafe.Add(ref dest, (nint)i + 7) = value;
// broadcast the value to all 8 bytes of the ulong and write it to memory
Unsafe.WriteUnaligned(ref Unsafe.Add(ref dest, i), value * 0x101010101010101ul);
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it's not just for Mono, but it's shorter (and faster)

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This is 64-bit multiplication. Is it faster on 32-bit platforms as well?

} while ((i += 8) < stopLoopAtOffset);
}

// Write next 4 elements if needed
if ((len & 4) != 0)
{
Unsafe.Add(ref dest, (nint)i + 0) = value;
Unsafe.Add(ref dest, (nint)i + 1) = value;
Unsafe.Add(ref dest, (nint)i + 2) = value;
Unsafe.Add(ref dest, (nint)i + 3) = value;
// broadcast the value to all 4 bytes of the uint and write it to memory
Unsafe.WriteUnaligned(ref Unsafe.Add(ref dest, i), value * 0x1010101u);
i += 4;
}

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