Perf: JIT sometimes does not elide duplicate checks across inlined methods #69080
Description
If two methods which call Span<T>.IsEmpty are both inlined into the caller, JIT will not properly deduplicate the second check. See repro and codegen below.
Repro code
public class C
{
public static void M(ReadOnlySpan<Char> s)
{
if (s.StartsWith('\"') && s.EndsWith('\"'))
{
Console.WriteLine("Quoted!");
}
}
}
public static class Extensions
{
public static bool StartsWith(this ReadOnlySpan<Char> span, char c)
{
if (span.IsEmpty) { return false; }
if (span[0] == c) { return true; }
return false;
}
public static bool EndsWith(this ReadOnlySpan<Char> span, char c)
{
if (span.IsEmpty) { return false; }
nuint localLength = (uint)span.Length;
if (Unsafe.Add(ref MemoryMarshal.GetReference(span), (nint)localLength - 1) == c) { return true; }
return false;
}
}; Method ConsoleApp7.C:M(System.ReadOnlySpan`1[Char])
G_M60782_IG01:
sub rsp, 40
;; size=4 bbWeight=1 PerfScore 0.25
G_M60782_IG02:
mov rax, bword ptr [rcx]
mov rdx, rax
mov ecx, dword ptr [rcx+8]
mov r8d, ecx
test r8d, r8d
je SHORT G_M60782_IG04
;; size=17 bbWeight=1 PerfScore 5.75
G_M60782_IG03:
cmp word ptr [rdx], 34
jne SHORT G_M60782_IG04
test ecx, ecx ; <-- ** duplicate span.IsEmpty check **
je SHORT G_M60782_IG04
mov ecx, ecx
cmp word ptr [rax+2*rcx-2], 34
jne SHORT G_M60782_IG04
mov rcx, 0xD1FFAB1E ; "Quoted!"
mov rcx, gword ptr [rcx]
call [System.Console:WriteLine(System.String)]
;; size=39 bbWeight=0.50 PerfScore 7.38
G_M60782_IG04:
nop
;; size=1 bbWeight=1 PerfScore 0.25
G_M60782_IG05:
add rsp, 40
ret
;; size=5 bbWeight=1 PerfScore 1.25
; Total bytes of code: 66Interestingly, if the signatures of both extension methods are changed from this to in this, the second check is correctly elided.
G_M60782_IG02:
mov rax, bword ptr [rcx]
mov ecx, dword ptr [rcx+8]
test ecx, ecx
je SHORT G_M60782_IG04
;; size=10 bbWeight=1 PerfScore 5.25
G_M60782_IG03:
cmp word ptr [rax], 34
jne SHORT G_M60782_IG04
mov ecx, ecx
cmp word ptr [rax+2*rcx-2], 34
jne SHORT G_M60782_IG04
mov rcx, 0xD1FFAB1E ; "Quoted!"
mov rcx, gword ptr [rcx]
call [System.Console:WriteLine(System.String)]category:performance
theme:inlining
skill-level:intermediate
cost:medium
impact:small