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Redundant bound check in monotonically decreasing loop #135008
Description
From #134956 (comment)
struct Test
{
uint[] _bitmap;
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static bool Contains(uint[] bitmap, int value)
{
uint offset = (uint)(value >> 5);
return offset < (uint)bitmap.Length && (bitmap[offset] & (1u << value)) != 0;
}
private int LastIndexOfAny(ReadOnlySpan<char> span)
{
uint[] bitmap = _bitmap;
int searchSpaceLength = span.Length;
while (--searchSpaceLength >= 0)
{
char c = span[searchSpaceLength];
if (Contains(bitmap, c))
{
break;
}
}
return searchSpaceLength;
}
}Test:LastIndexOfAny(System.ReadOnlySpan`1[char]):int:this (FullOpts):
push rbp
mov rbp, rsp
mov rax, gword ptr [rdi]
lea ecx, [rdx-0x01]
mov edi, ecx
test edi, edi
jl SHORT G_M5479_IG06
align [0 bytes for IG03]
G_M5479_IG03: ;; offset=0x0010
cmp ecx, edx
jae SHORT G_M5479_IG08
mov r8d, ecx
movzx r8, word ptr [rsi+2*r8]
mov r9d, r8d
sar r9d, 5
mov r10d, dword ptr [rax+0x08]
cmp r10d, r9d
jbe SHORT G_M5479_IG05
mov r9d, r9d
mov r9d, dword ptr [rax+4*r9+0x10]
bt r9d, r8d
jb SHORT G_M5479_IG06
G_M5479_IG05: ;; offset=0x003A
lea ecx, [rdi-0x01]
mov edi, ecx
test edi, edi
jge SHORT G_M5479_IG03
G_M5479_IG06: ;; offset=0x0043
mov eax, ecx
pop rbp
ret
G_M5479_IG08: ;; offset=0x0047
call CORINFO_HELP_RNGCHKFAIL
int3 Reacted by Julian and Miha Zupan
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dotnet-policy-service commentedon Oct 1, 2026
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