JIT: Coalesce adjacent constant-address stores

[EgorBo]

Extend LowerStoreCoalescing to recognize STOREIND nodes whose address is an absolute (non-relocatable) constant, e.g. writes to adjacent fields of a non-GC static struct:

private struct S2 { public int A, B; }

private static S2 s_static2;

[MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
private static void Test()
{
    s_static2.A = 1;
    s_static2.B = 2;
}

Codegen for Test on win-x64:

-       mov      rax, 0x1E049543570      ; data for Program:s_static2
-       mov      dword ptr [rax], 1
-       mov      rax, 0x1E049543574
-       mov      dword ptr [rax], 2
+       mov      rax, 0x200000001
+       mov      rcx, 0x1F549543570
+       mov      qword ptr [rcx], rax

Diffs

Basically, I represent it as baseAddr being null and offset being the full absolute address, the rest of the code is the same, just had to bump the type of offset from int to ssize_t

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[EgorBo]

@MihuBot -nuget

[Copilot]

Pull request overview

This PR extends JIT lowering-time store coalescing so that adjacent STOREIND nodes whose address is a non-relocatable absolute constant can be recognized as targeting the same “base” and then merged into a single wider store, while updating offset-related arithmetic/alignment checks to support that new address form.

Changes:

• Teach GetLoadStoreCoalescingData to treat non-relocatable constant addresses as (base=null, offset=<absolute address>) so adjacent constant-address stores can be matched/coalesced.
• Widen LoadStoreCoalescingData::offset to ssize_t and adjust related offset math, alignment, and atomicity checks to account for baseAddr == nullptr.
• Update the store-coalescing rewrite path to correctly update either GT_LEA offsets or constant-address GT_CNS_INT values after coalescing.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/jit/lower.h	Widen store-coalescing offset storage to ssize_t.
src/coreclr/jit/lower.cpp	Add constant-address handling for coalescing + adjust atomicity/alignment and rewrite logic accordingly.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[EgorBo]

PTAL @AndyAyersMS as someone who recently touched this area, cc @dotnet/jit-contrib