Loop condition vs initial guard check impacts bounds checking #83349
Description
Consider these two functionally-identical loops:
public static int M1(int i, ReadOnlySpan<char> src)
{
int sum = 0;
while ((uint)i < (uint)src.Length)
{
sum += src[i++];
}
return sum;
}
public static int M2(int i, ReadOnlySpan<char> src)
{
int sum = 0;
while (true)
{
if ((uint)i >= (uint)src.Length) break;
sum += src[i++];
}
return sum;
}The second simply moves the loop condition to be the very first thing in the body. SharpLab even decompiles them to C# identically. However, the former has bounds checks whereas they're appropriately removed for the latter:
SharpLab
C.M1(Int32, System.ReadOnlySpan`1<Char>)
L0000: sub rsp, 0x28
L0004: mov rax, [rdx]
L0007: mov edx, [rdx+8]
L000a: xor r8d, r8d
L000d: cmp ecx, edx
L000f: jae short L0039
L0011: nop [rax]
L0018: nop [rax+rax]
L0020: lea r9d, [rcx+1]
L0024: cmp ecx, edx
L0026: jae short L0041
L0028: mov ecx, ecx
L002a: movzx ecx, word ptr [rax+rcx*2]
L002e: add r8d, ecx
L0031: cmp r9d, edx
L0034: mov ecx, r9d
L0037: jb short L0020
L0039: mov eax, r8d
L003c: add rsp, 0x28
L0040: ret
L0041: call 0x00007fff38258b30
L0046: int3
C.M2(Int32, System.ReadOnlySpan`1<Char>)
L0000: mov rax, [rdx]
L0003: mov edx, [rdx+8]
L0006: xor r8d, r8d
L0009: cmp ecx, edx
L000b: jae short L001f
L000d: lea r9d, [rcx+1]
L0011: mov ecx, ecx
L0013: movzx ecx, word ptr [rax+rcx*2]
L0017: add r8d, ecx
L001a: mov ecx, r9d
L001d: jmp short L0009
L001f: mov eax, r8d
L0022: ret