[Tex Riddell (tex3d)]

This subtle bug can cause a failure to resolve a static resource in a corner case where DxilValueCache could replace an odd-indexed phi, and this is needed to resolve other phis to known values, but an indexing bug prevents the loop from ever visiting any phi nodes at odd-numbered indexes in the list.

This change also adds two IR-based pass tests for this code path, as there weren't any before.

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[Copilot]

🟢 Approval recommended

The localized traversal fix handles erasure and termination correctly, with targeted regression coverage and no identified blocking issues.

No open issues 🎉

What changed in this PR

Fixes resource legalization skipping PHI nodes at odd-numbered indexes, allowing dependent PHIs to resolve to known resources.

Changes:

• Replaces index-based traversal with an iterator loop that safely handles erasure.
• Repeats simplification until all PHIs are resolved or no progress is made.
• Adds two IR regression tests covering resource resolution, including the dependent-PHI corner case.

File	Description
tools/​clang/​test/​DXC/​Passes/​DxilLowerCreateHandleForLib/​legalize-resource-second-phi-first.ll	Tests resolution that depends on visiting the second PHI first.
tools/​clang/​test/​DXC/​Passes/​DxilLowerCreateHandleForLib/​legalize-resource-phi.ll	Tests basic PHI resource resolution and resource metadata cleanup.
lib/​HLSL/​DxilCondenseResources.cpp	Fixes PHI traversal and preserves progress-based termination.

_{Review effort: Balanced}

[Deric C. (Icohedron)]

The algorithm looks correct to me.

The pseudocode for the core part of this change seems to be:

-for (Attempt = 0; Attempt < PHIs.size(); Attempt++)
-  for (i = 0; i < PHIs.size(); i++)
-    if GetValue(PHIs[i]): RAUW, erase(i)   // header i++ skips the shifted PHI
-    else:                 i++              // header i++ skips the next PHI
-  if no change: break                      // odd slots never tried
+while PHIs not empty
+  for each It in PHIs
+    if GetValue(*It): RAUW, It = erase(It)
+    else:             ++It
+  if no change: break                      // every pass visits every PHI

One additional review comment I have is that the PR description states:

... an indexing bug prevents the loop from ever visiting any phi nodes at odd-numbered indexes in the list.

but that isn't precisely correct. It should instead read something like:

If no phi node at an even position resolves in a pass/attempt, the loop exits without trying phi nodes at odd positions.

[Deric C. (Icohedron)]

nit: There should be a comment stating that this loop intentionally re-evaluates .end() every iteration.

LLVM Coding Standards say to not evaluate .end() every iteration of the loop. But in this case it is necessary because the container is being mutated, and therefore a comment is required to state that it is intentional.
https://llvm.org/docs/CodingStandards.html#don-t-evaluate-end-every-time-through-a-loop

[Deric C. (Icohedron)]

Though the coding standards does also say to prefer range-based for-loops I do agree this is easier to follow using an iterator-based loop.

[Deric C. (Icohedron)]

typo

Suggested change

	case1: ; preds = loopback, %entry
	case1: ; preds = %loopback, %entry