Fill uninitialized address-taken locals with poison pattern in debug codegen

[jkotas]

C/C++ compilers have this debugging/diagnostic aid for years.

More context: dotnet/coreclr#25674 (comment)

Related to: dotnet/csharplang#868

category:testing
theme:testing
skill-level:beginner
cost:small

[GrabYourPitchforks]

Honestly, I'd like to see this not just for debug codegen, but also as an opt-in thing that admins could enable in production applications. Going to my cloud hosting portal and hitting a checkbox that says "my app is misbehaving; please enable additional error checking at runtime" is a compelling scenario.

[xtqqczze]

runtime/src/coreclr/tools/Common/JitInterface/CorInfoImpl.cs

Lines 2755 to 2760 in 907fc3e

	#if DEBUG
	// In debug, write some bogus data to the struct to ensure we have filled everything
	// properly.
	fixed (CORINFO_EE_INFO* tmp = &pEEInfoOut)
	MemoryHelper.FillMemory((byte*)tmp, 0xcc, Marshal.SizeOf<CORINFO_EE_INFO>());
	#endif

runtime/src/coreclr/tools/aot/ILCompiler.ReadyToRun/JitInterface/CorInfoImpl.ReadyToRun.cs

Lines 568 to 573 in dcb42e8

	#if DEBUG
	// In debug, write some bogus data to the struct to ensure we have filled everything
	// properly.
	fixed (CORINFO_LOOKUP* tmp = &pLookup)
	MemoryHelper.FillMemory((byte*)tmp, 0xcc, sizeof(CORINFO_LOOKUP));
	#endif

also L1210-L1214, L1385-L1389

runtime/src/libraries/System.Security.Cryptography.Pkcs/src/Microsoft/Win32/SafeHandles/SafeHeapAllocHandle.cs

Lines 23 to 33 in 01b7e73

	#if DEBUG
	result._size = size;
	unsafe
	{
	byte* p = (byte*)(result.DangerousGetHandle());
	for (int i = 0; i < size; i++)
	{
	p[i] = 0xcc;
	}
	}
	#endif

also: L39-L48

runtime/src/libraries/System.Private.CoreLib/src/System/Number.NumberBuffer.cs

Lines 43 to 45 in 01b7e73

	#if DEBUG
	Digits.Fill(0xCC);
	#endif

[xtqqczze]

runtime/src/libraries/System.Formats.Asn1/src/System/Formats/Asn1/AsnWriter.cs

Lines 270 to 273 in 753b764

	#if DEBUG
	// Ensure no "implicit 0" is happening, in case we move to pooling.
	_buffer.AsSpan(_offset).Fill(0xCA);
	#endif