C# value discard leads to range check for happy path #99943
Description
Description
Code:
#nullable disable
using System;
using System.Runtime.CompilerServices;
public class C {
public static void Main() {
Console.WriteLine(System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription);
M(null);
}
[MethodImpl(MethodImplOptions.NoInlining)]
public static int M(byte [] a) {
for (int i = 0; i < 10; ++i) {
_ = a[i];
Console.WriteLine(a[i]);
}
return 5;
}
}running with $env:DOTNET_JitDisasm="M", $env:DOTNET_TieredCompilation=0
Generated code
.NET 9.0.0-preview.2.24128.5
; Assembly listing for method C:M(ubyte[]):int (FullOpts)
; Emitting BLENDED_CODE for X64 with AVX - Windows
; FullOpts code
; optimized code
; rsp based frame
; partially interruptible
; No PGO data
G_M000_IG01: ;; offset=0x0000
push rdi
push rsi
push rbx
sub rsp, 32
mov rbx, rcx
G_M000_IG02: ;; offset=0x000A
xor esi, esi
test rbx, rbx ;; I don't quite understand for what this null check is for, since it jumps to the same loop, but it's another issue
je SHORT G_M000_IG06
G_M000_IG03: ;; offset=0x0011
mov edi, dword ptr [rbx+0x08]
cmp edi, 10
jl SHORT G_M000_IG06
G_M000_IG04: ;; offset=0x0019
cmp esi, edi ;; range check on happy path :(
jae SHORT G_M000_IG09
mov ecx, esi
movzx rcx, byte ptr [rbx+rcx+0x10]
call [System.Console:WriteLine(int)]
inc esi
cmp esi, 10
jl SHORT G_M000_IG04
G_M000_IG05: ;; offset=0x0031
jmp SHORT G_M000_IG07
G_M000_IG06: ;; offset=0x0033
mov edi, dword ptr [rbx+0x08]
cmp esi, edi
jae SHORT G_M000_IG09
mov ecx, esi
movzx rcx, byte ptr [rbx+rcx+0x10]
call [System.Console:WriteLine(int)]
inc esi
cmp esi, 10
jl SHORT G_M000_IG06
G_M000_IG07: ;; offset=0x004E
mov eax, 5
G_M000_IG08: ;; offset=0x0053
add rsp, 32
pop rbx
pop rsi
pop rdi
ret
G_M000_IG09: ;; offset=0x005B
call CORINFO_HELP_RNGCHKFAIL
int3
; Total bytes of code 97There is G_M000_IG04 section is identical to G_M000_IG06, both contain range check despite the fact that G_M000_IG03 should guard against it.
Using a[i] normally (i.e. second System.Console.WriteLine(a[i]); correctly omits range check.
Correct example
For
for (int i = 0; i < 10; ++i) {
Console.WriteLine(a[i]);
Console.WriteLine(a[i]);
}Generated code (excerpt)
G_M000_IG03: ;; offset=0x0010
cmp dword ptr [rbx+0x08], 10
jl SHORT G_M000_IG06
G_M000_IG04: ;; offset=0x0016
mov ecx, esi
movzx rcx, byte ptr [rbx+rcx+0x10]
call [System.Console:WriteLine(int)]
mov ecx, esi
movzx rcx, byte ptr [rbx+rcx+0x10]
call [System.Console:WriteLine(int)]
inc esi
cmp esi, 10
jl SHORT G_M000_IG04
G_M000_IG05: ;; offset=0x0037
jmp SHORT G_M000_IG07
G_M000_IG06: ;; offset=0x0039
cmp esi, dword ptr [rbx+0x08]
jae SHORT G_M000_IG09
mov ecx, esi
movzx rcx, byte ptr [rbx+rcx+0x10]
call [System.Console:WriteLine(int)]
mov ecx, esi
movzx rcx, byte ptr [rbx+rcx+0x10]
call [System.Console:WriteLine(int)]
inc esi
cmp esi, 10
jl SHORT G_M000_IG06