Intrinsify string.FastAllocateString

[EgorBo]

Another experiment (for better #127655)

Mark String.FastAllocateString with [Intrinsic] and teach RyuJIT to:

• Track the result as known non-null via VNF_StrFastAllocate (knownNonNull=true)
• Fold ARR_LENGTH(VNF_StrFastAllocate(len)) -> len
• Emit a global assertion 'string.Length(result) == lenArg' in optAssertionGen

Example:

int Foo()
{
    string a = "aaa";
    string b = "bbb";
    string c = a + b;
    return c[3]; // this PR removes this bound check
}

[EgorBo]

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[EgorBo]

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Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

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Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

[EgorBo]

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Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

src/coreclr/jit/valuenum.cpp:2576

• This change introduces new JIT intrinsic/VN behavior that affects bounds-check elimination (e.g. folding ARR_LENGTH(StrFastAllocate(...))). Please add a JIT regression test that verifies the expected optimization (for example, the concat+index pattern from the PR description, with a disasm check or other stable assertion) so we can catch future regressions.

            // Case 4: ARR_LENGTH(new T[(long)size]) -> size
            //         ARR_LENGTH(String.FastAllocateString(pMT, (long)size)) -> size
            VNFuncApp newArrFuncApp;
            if (GetVNFunc(arg0VN, &newArrFuncApp) &&
                ((newArrFuncApp.m_func == VNF_JitNewArr) || (newArrFuncApp.m_func == VNF_StrFastAllocate)))
            {
                ValueNum actualSizeVN = VNIgnoreIntToLongCast(newArrFuncApp.m_args[1]);
                if (TypeOfVN(actualSizeVN) == TYP_INT)
                {
                    *resultVN = actualSizeVN;
                }