Extra bounds checks are being generated in common span usage pattern #10950
Description
I would expect the following code to have no bounds checks within the disassembly beyond the explicit length check, when accessing the data via the indexer:
public static bool IsFirstSpace(ReadOnlySpan<byte> buffer)
{
if (buffer.Length < 1 || buffer[0] != ' ')
return false;
return true;
}However, I am seeing an extra comparison and jump instruction:
00007FFEC26EC790 sub rsp,28h
00007FFEC26EC794 mov rax,qword ptr [rcx]
00007FFEC26EC797 mov edx,dword ptr [rcx+8]
00007FFEC26EC79A test edx,edx
00007FFEC26EC79C jle 00007FFEC26EC7A8
00007FFEC26EC79E cmp edx,0
00007FFEC26EC7A1 jbe 00007FFEC26EC7B9
00007FFEC26EC7A3 cmp byte ptr [rax],20h
00007FFEC26EC7A6 je 00007FFEC26EC7AF
00007FFEC26EC7A8 xor eax,eax
00007FFEC26EC7AA add rsp,28h
00007FFEC26EC7AE ret
00007FFEC26EC7AF mov eax,1
00007FFEC26EC7B4 add rsp,28h
00007FFEC26EC7B8 ret
00007FFEC26EC7B9 call 00007FFF214A56F0
00007FFEC26EC7BE int 3
Workarounds like casting to uint, etc. do not help:
public static bool IsFirstSpace(ReadOnlySpan<byte> buffer)
{
if ((uint)buffer.Length >= 1u && buffer[0] == ' ')
return true;
return false;
}
public static bool IsFirstSpace(ReadOnlySpan<byte> buffer)
{
if (!buffer.IsEmpty && buffer[0] == ' ')
return true;
return false;
}Version: Microsoft.NETCore.App 3.0.0-preview1-26809-04
cc @AndyAyersMS, @benaadams, @GrabYourPitchforks
category:cq
theme:range-check
skill-level:expert
cost:large