[interp] Fold "ldstr".Length to a constant

[EgorBo]

A similar optimization for RyuJIT was quite profitable judging by the jit-diff: #1378

Example:

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

[ghost]

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[BrzVlad]

In which real world scenario does this provide us with an improvement ? Especially since, unlike ryu-jit I assume, we don't even track the string literal across variables, so it wouldn't be able to optimize the example from the linked PR, using the Validate method.

[EgorBo]

In which real world scenario does this provide us with an improvement ? Especially since, unlike ryu-jit I assume, we don't even track the string literal across variables, so it wouldn't be able to optimize the example from the linked PR, using the Validate method.

even without inlining like in ryujit PR (the idea of that sample was to answer the question why that optimization was not done in Roslyn) it's still useful IMO, search for ".Length in the repo (and there are more results where SOME_STR_CONST.Length is used).

[EgorBo]

Tests fail with

    System.Linq.Tests.TakeWhileTests.IndexTakeWhileOverflowBeyondIntMaxValueElements [SKIP]
      Condition(s) not met: "IsStressModeEnabled"
* Assertion at /__w/1/s/src/mono/mono/mini/interp/transform.c:7903, condition `sp >= stack && sp <= stack_end' not met

will fix.