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Jit doesn't recognise ldstr as non-null #12011
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I think this can be even further optimized - the JIT should know the length of the string being loaded so can burn it in to the code gen. There's no need to query the string length and move it to the appropriate location. Sample: mov rax, 123456h ; whatever the known entry index is that corresponds to this string
mov rax, qword ptr [rax] ; rax now contains the string object reference itself
mov rax, qword ptr [rax + 8h] ; rax now contains a reference to the beginning of the string data
mov qword ptr [rdx], rax ; [rdx] = Span.Pointer
mov dword ptr [rdx + 8h], 5 ; [rdx + 8h] = Span.Length, known length of fixed string "Hello" |
Eh, G_M55889_IG01:
0F1F440000 nop
G_M55889_IG02:
48B85033C1EA24020000 mov rax, 0x224EAC13350
488B00 mov rax, gword ptr [rax]
8B10 mov edx, dword ptr [rax]
488BD0 mov rdx, rax
3912 cmp dword ptr [rdx], edx
4883C20C add rdx, 12
8B4008 mov eax, dword ptr [rax+8]
488911 mov bword ptr [rcx], rdx
894108 mov dword ptr [rcx+8], eax
488BC1 mov rax, rcx
G_M55889_IG03:
C3 ret Still need to get rid of a NULLCHECK and propagate the constant string length. |
From a register point of view would swapping the arguments to the span .ctor help any?
to
Since the layout of strings and arrays is |
In the constant case I don't see why it would matter.
That seems to be unexpectedly complicated, the VM keeps the JIT in the dark about the string length. The JIT interface will probably need to be extended. |
I've run across the idea of allowing the jit to know the length and even the contents of Once the string is materialized the jit can also inspect it, so we can do similar things with readonly static strings. Can we tie this to some motivating cases in Kestrel? |
With (both) nullchecks removed: 48B85033766563020000 mov rax, 0x26365763350
488B00 mov rax, gword ptr [rax]
488D500C lea rdx, bword ptr [rax+12]
8B4008 mov eax, dword ptr [rax+8]
488911 mov bword ptr [rcx], rdx
894108 mov dword ptr [rcx+8], eax
488BC1 mov rax, rcx With some horrible hack to get the string length: 48B85033E92E1A020000 mov rax, 0x21A2EE93350
488B00 mov rax, gword ptr [rax]
4883C00C add rax, 12
BA09000000 mov edx, 9
488901 mov bword ptr [rcx], rax
895108 mov dword ptr [rcx+8], edx
488BC1 mov rax, rcx Interestingly, this shows a problem with constant propagation. So, null checks are pretty easy to get rid of during import/inline/morph. Might be interesting/useful to also support null check removal in VN but it's not clear how to do that because by the time we reach VN GT_CNS_STR nodes are gone, replaced by indirections and handles. Dealing with constant lengths, well, that's another story. |
Do these static readonly string sm_str = "default";
static X()
{
if (someRuntimeCondition())
{
sm_str = "overwritten";
}
} (i was thinking if |
Can be closed I guess (via #1378 + #37245) static ReadOnlySpan<char> ToSpan() => "literal"; ; Method ToSpan():System.ReadOnlySpan`1[Char]
G_M54852_IG01:
;; bbWeight=1 PerfScore 0.00
G_M54852_IG02:
mov rax, 0xD1FFAB1E
mov rax, gword ptr [rax]
add rax, 12
mov bword ptr [rcx], rax
mov dword ptr [rcx+8], 7
mov rax, rcx
;; bbWeight=1 PerfScore 4.75
G_M54852_IG03:
ret
;; bbWeight=1 PerfScore 1.00
; Total bytes of code: 31 |
/cc @GrabYourPitchforks
/cc @KrzysztofCwalina as you mentioned in dotnet/roslyn#24621 (comment)
category:cq
theme:basic-cq
skill-level:expert
cost:large
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