[Assignment Tracking] Skip scalable vectors in declare-to-assign pass

[Orlando]

Summary

Do not convert dbg.declares to dbg.assigns for variables backed by scalable vector allocas as this isn't yet supported.

[jmorse]

LGTM

llvm/lib/IR/DebugInfo.cpp

 }
 
 bool AssignmentTrackingPass::runOnFunction(Function &F) {
   // No value in assignment tracking without optimisations.
   if (F.hasFnAttribute(Attribute::OptimizeNone))
     return /*Changed*/ false;
 
   bool Changed = false;
+  auto *DL = &F.getParent()->getDataLayout();
   // Collect a map of {backing storage : dbg.declares} (currently "backing
   // storage" is limited to Allocas). We'll use this to find dbg.declares to
   // delete after running `trackAssignments`.
   DenseMap<const AllocaInst *, SmallPtrSet<DbgDeclareInst *, 2>> DbgDeclares;
   // Create another similar map of {storage : variables} that we'll pass to
   // trackAssignments.
   StorageToVarsMap Vars;
   for (auto &BB : F) {
     for (auto &I : BB) {
       DbgDeclareInst *DDI = dyn_cast<DbgDeclareInst>(&I);
       if (!DDI)
         continue;
       // FIXME: trackAssignments doesn't let you specify any modifiers to the
       // variable (e.g. fragment) or location (e.g. offset), so we have to
       // leave dbg.declares with non-empty expressions in place.
       if (DDI->getExpression()->getNumElements() != 0)
         continue;
       if (!DDI->getAddress())
         continue;
       if (AllocaInst *Alloca =
               dyn_cast<AllocaInst>(DDI->getAddress()->stripPointerCasts())) {
         // FIXME: Skip VLAs for now (let these variables use dbg.declares).
         if (!Alloca->isStaticAlloca())
           continue;
+        // Similarly, skip scalable vectors (use dbg.declares instead).
+        if (auto Sz = Alloca->getAllocationSize(*DL); Sz && Sz->isScalable())
+          continue;
         DbgDeclares[Alloca].insert(DDI);
         Vars[Alloca].insert(VarRecord(DDI));
       }
     }
   }
 
-  auto DL = std::make_unique<DataLayout>(F.getParent());
   // FIXME: Locals can be backed by caller allocas (sret, byval).
   // Note: trackAssignments doesn't respect dbg.declare's IR positions (as it
   // doesn't "understand" dbg.declares). However, this doesn't appear to break
   // any rules given this description of dbg.declare from
   // llvm/docs/SourceLevelDebugging.rst:
   //
   //   It is not control-dependent, meaning that if a call to llvm.dbg.declare
   //   exists and has a valid location argument, that address is considered to

llvm/test/DebugInfo/Generic/assignment-tracking/declare-to-assign/scalable-vector.ll

+; RUN: opt -passes=declare-to-assign %s -S | FileCheck %s
+
+;; Check declare-to-assign skips scalable vectors for now. i.e. do not replace
+;; the dbg.declare with a dbg.assign intrinsic.
+
+; CHECK: call void @llvm.dbg.declare(metadata ptr %c
+
+define dso_local void @b() !dbg !9 {
+entry:
+  %c = alloca <vscale x 8 x i32>, align 4
+  call void @llvm.dbg.declare(metadata ptr %c, metadata !13, metadata !DIExpression()), !dbg !21
+  ret void
+}
+
+declare void @llvm.dbg.declare(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3, !5, !7}
+!llvm.ident = !{!8}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, producer: "clang version 17.0.0", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.c", directory: "/")
+!2 = !{i32 7, !"Dwarf Version", i32 5}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"target-abi", !"lp64"}
+!7 = !{i32 8, !"SmallDataLimit", i32 8}
+!8 = !{!"clang version 17.0.0"}
+!9 = distinct !DISubprogram(name: "b", scope: !1, file: !1, line: 2, type: !10, scopeLine: 2, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !12)
+!10 = !DISubroutineType(types: !11)
+!11 = !{null}
+!12 = !{}
+!13 = !DILocalVariable(name: "c", scope: !9, file: !1, line: 2, type: !14)
+!14 = !DIDerivedType(tag: DW_TAG_typedef, name: "a", file: !1, line: 1, baseType: !15)
+!15 = !DIDerivedType(tag: DW_TAG_typedef, name: "__rvv_uint32m4_t", file: !16, baseType: !17)
+!16 = !DIFile(filename: "test.c", directory: "/")
+!17 = !DICompositeType(tag: DW_TAG_array_type, baseType: !18, flags: DIFlagVector, elements: !19)
+!18 = !DIBasicType(name: "unsigned int", size: 32, encoding: DW_ATE_unsigned)
+!19 = !{!20}
+!20 = !DISubrange(lowerBound: 0, upperBound: !DIExpression(DW_OP_bregx, 7202, 0, DW_OP_constu, 4, DW_OP_div, DW_OP_constu, 4, DW_OP_mul, DW_OP_constu, 1, DW_OP_minus))
+!21 = !DILocation(line: 2, column: 14, scope: !9)
+