JIT: Generalize pattern-based IV analysis

[jakobbotsch]

• Follow unique preds when locating the init statement for the iterator variable
• Generalize zero-trip detection by similarly following unique preds

This at least allows us to analyze the loop iteration here: #119277 (comment)

However, we still do not unroll because we bail out on this condition in unrolling:

runtime/src/coreclr/jit/optimizer.cpp

Lines 1347 to 1358 in dcf8680

	// The loop test must be both an exit and a backedge.
	// FlowGraphNaturalLoop::AnalyzeIteration ensures it is an exit but we must
	// make sure it is a backedge so that we can legally redirect it to the
	// next iteration. If it isn't a backedge then redirecting it would skip
	// all code between the loop test and the backedge.
	assert(loop->ContainsBlock(iterInfo.TestBlock->GetTrueTarget()) !=
	loop->ContainsBlock(iterInfo.TestBlock->GetFalseTarget()));
	if (!iterInfo.TestBlock->TrueTargetIs(loop->GetHeader()) && !iterInfo.TestBlock->FalseTargetIs(loop->GetHeader()))
	{
	JITDUMP("Failed to unroll loop " FMT_LP ": test block is not a backedge\n", loop->GetIndex());
	return false;
	}

I think we truly do need to improve heuristics for inversion to fix the case.

[Copilot]

Pull Request Overview

This PR generalizes pattern-based induction variable (IV) analysis in the JIT compiler to improve loop optimization capabilities. The changes focus on following unique predecessors when locating initialization statements and generalizing zero-trip detection.

Key changes:

• Remove init extraction from optExtractInitTestIncr and create a new focused method optExtractTestIncr
• Add new methods FindConstInit and HasZeroTripTest that follow unique predecessors to locate initialization and zero-trip tests
• Add gtTreeHasLocalStore utility to check for stores affecting specified locals

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/coreclr/jit/optimizer.cpp	Refactored to remove init extraction logic from optExtractInitTestIncr, renamed to optExtractTestIncr
src/coreclr/jit/gentree.cpp	Added new gtTreeHasLocalStore method to detect stores affecting a local variable
src/coreclr/jit/flowgraph.cpp	Added FindConstInit and HasZeroTripTest methods, removed old init matching logic
src/coreclr/jit/compiler.h	Updated method signatures and added new method declarations

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @AndyAyersMS

Diffs. This mostly results in more loops we can unroll. Comparing on benchmarks.run_pgo, we go from 1443 cloned, 260 unrolled to 1445 cloned, 342 unrolled with this change.