Closed
Description
Consider a method like:
private static bool M(int pos) {
const string Str = "hello";
return (uint)pos < Str.Length && Str[pos] == 'e';
}The JIT is currently generating code like:
G_M23807_IG01:
4883EC28 sub rsp, 40
;; bbWeight=1 PerfScore 0.25
G_M23807_IG02:
48B8C0300090BF020000 mov rax, 0x2BF900030C0
488B00 mov rax, gword ptr [rax]
8B5008 mov edx, dword ptr [rax+8]
4863D2 movsxd rdx, edx
448BC1 mov r8d, ecx
493BD0 cmp rdx, r8
7E19 jle SHORT G_M23807_IG05
;; bbWeight=1 PerfScore 6.00
G_M23807_IG03:
3B4808 cmp ecx, dword ptr [rax+8]
731B jae SHORT G_M23807_IG07
4863D1 movsxd rdx, ecx
66837C500C65 cmp word ptr [rax+2*rdx+12], 101
0F94C0 sete al
0FB6C0 movzx rax, al
;; bbWeight=0.50 PerfScore 3.25
G_M23807_IG04:
4883C428 add rsp, 40
C3 ret
;; bbWeight=0.50 PerfScore 0.63
G_M23807_IG05:
33C0 xor eax, eax
;; bbWeight=0.50 PerfScore 0.13
G_M23807_IG06:
4883C428 add rsp, 40
C3 ret
;; bbWeight=0.50 PerfScore 0.63
G_M23807_IG07:
E8DC5CC95F call CORINFO_HELP_RNGCHKFAIL
CC int3
In theory it seems the JIT should be able to both a) remove the redundant bounds check (and the corresponding fallback to rngchkfail), and b) encode the string length as a constant rather than reading it from the object.
cc: @AndyAyersMS
category:cq
theme:bounds-checks
skill-level:intermediate
cost:medium
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