JIT: Optimize "constant_string".Length #1378
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hm.. "regression" example: static StringBuilder Tess()
{
return new StringBuilder("hello");
}Diff (new codegen is on the right): https://www.diffchecker.com/QzLuLcFj |
- mov r9d, dword ptr [rcx+8]
- ;; bbWeight=0.25 PerfScore 0.50
+ mov r9d, 5
+ ;; bbWeight=0.25 PerfScore 0.06
...
- ; Total bytes of code: 81
+ ; Total bytes of code: 83this is because immediate value for mov is always 32 bit (and there is no 8 bit counterpart, imm8), so the instruction takes 6 bytes. otoh, if it instead emits: push 5
pop r9 the diff will go away (for same range 0:127 with both masm and nasm). |
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@am11 On x64 (windows, at least) we generally can't change |
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Other option was to use |
I wonder how many of these come from inlining? The jit inliner currently doesn't consider a constant string passed as an argument to be noteworthy. Perhaps it should (or should, if that arg ends up in a bounds check...).
Is this representative of what causes the actual regressions? I realize there aren't that many but wonder if anything else is going on that we should investigate. |
looks so, here is the biggest "regression" (Microsoft.CodeAnalysis.CSharp.Binder:BindIntegralMinValConstants): https://www.diffchecker.com/5ttTCfO4
If I read the diff correctly - none of them were inlined but maybe I am wrong. I understand what you mean but I guess tweaking the jit heuristics (better multiplier for cns strings) is too complicated for this PR 🙂 |
Looks like we are now missing some CSEs and so re-fetching strings from their handles, which is what leads to the size increase. Can you investigate?
You could run diffs with inlining disabled (say via |
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@AndyAyersMS jit-diff results with see https://gist.github.com/EgorBo/4110b359ed109ea4ef198250aee3d3da so it's -30053 (default) diff vs -804 (noinline). Should I try to check for |
```csharp
static int Test() => "Hello".Length; // replace with just `5`
```
Before:
```asm
movabs rax,0x7fd6c1758160
mov eax,DWORD PTR [rax]
ret
```
Now:
```asm
mov eax,0x5
ret
```
Also, handles cases after inlining, e.g.:
```csharp
bool Validate(string str)
{
return str.Length > 0 && str.Length <= 100;
}
bool Test()
{
return Validate("Hello"); // Validate() will be inlined
}
```
New codegen for `Test`:
```asm
mov eax, 1 ; return true
ret
```
I have a similar PR for CoreCLR: dotnet#1378
jit-diff report found >30k bytes of improvements.
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Did you drop all changes in ThunkInput.txt by accident? |
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@jkotas oops, yes, reverted. |
The VM side of the changes LGTM (modulo one nit). |
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JIT/SPMI side of changes looks fine.
@BruceForstall here's another jit GUID bump. Do you want us to hold off merging this?
Could we hold off a couple weeks? And merge it approximately the same time as #2249? We're just today starting to get some long-awaited (internal only) SuperPMI collections that I'd like to be able to use before requiring a recollection. |
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@BruceForstall, can you please post back here when we can take this? Not sure how precise a "couple" was in your comments, and it's been two weeks 😄 Thanks! |
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We're ready to take JIT-EE interface changes now (some internal processes dependent on the JIT-EE interface GUID are now stable). I'll go ahead and merge this. |
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Thanks, Bruce. And Egor :) |
A similar optimization for RyuJIT was quite profitable judging by the jit-diff: dotnet/runtime#1378 Example: ```csharp static int Test() { return "hello".Length; } ``` Current `MONO_VERBOSE_METHOD=Test` output: ``` IR_0000: ldstr 0 IR_0002: strlen IR_0003: ret ``` New output: ``` IR_0000: ldc.i4.5 IR_0001: ret ```
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Resurrects dotnet/coreclr#26000
Example (showcases why it's better to have this opt in JIT rather than in Roslyn)
Current codegen for
Testmethod:New codegen for
Testmethod:The optimization is now implemented in
importer.cppinstead ofmorph.cpp(according to feedback). Also, didn't forget about crossgen2.Jit-diff (see report with
--count 1000here):In some rare cases this opt can brake bound check elimination over string literals but it doesn't work well even today (see #1343).