Fix perf regression: limit debug info to relevant subprograms (#8174) (#8177)

PR #7799 added debug info for global variables across all inlined
subprograms, creating O(subprograms x globals) debug instructions. This
caused 2-3x slower compilation for shaders with many globals and
deeply-inlined functions.

Fix: Instead of collecting ALL subprograms for a function (walking every
instruction), only collect subprograms from the alloca's actual users --
the instructions that reference the specific global variable. This
preserves the debug info feature (globals visible in inlined scopes that
use them) while eliminating work for unrelated scopes.

The 'Lower static global into Alloca' pass is 3.8x faster on a stress
test (41ms -> 11ms), and overall debug compilation is ~10% faster.

Fixes #8174

Co-authored-by: Copilot <[email protected]>

Author: damyanp

lib/Transforms/Scalar/ScalarReplAggregatesHLSL.cpp

@@ -6505,47 +6505,49 @@ ModulePass *llvm::createSROA_Parameter_HLSL() {
 namespace {
 
 struct GVDebugInfoPatchCache {
-  DenseMap<Function *, SetVector<DISubprogram *>> SubprogramsForFunction;
-  DenseSet<DILocation *> Seen;
+  DenseMap<DILocation *, DISubprogram *> LocToSubprogram;
+  DenseMap<Function *, DISubprogram *> FuncToSubprogram;
   DITypeIdentifierMap EmptyMap;
 
+  DISubprogram *GetSubprogramForLoc(DILocation *Loc) {
+    auto It = LocToSubprogram.find(Loc);
+    if (It != LocToSubprogram.end())
+      return It->second;
+    DISubprogram *Result = nullptr;
+    auto *Scope = dyn_cast<DIScope>(Loc->getScope());
+    while (Scope) {
+      if (auto SubP = dyn_cast<DISubprogram>(Scope)) {
+        Result = SubP;
+        break;
+      }
+      Scope = Scope->getScope().resolve(EmptyMap);
+    }
+    LocToSubprogram[Loc] = Result;
+    return Result;
+  }
+
+  // Collect DISubprograms from a DILocation's inlined-at chain.
   void CollectSubprograms(DILocation *Loc, SetVector<DISubprogram *> &Set) {
     while (Loc) {
-      // This is potentially very expensive. Avoid repeatedly looking for
-      // DISubprogram's
-      if (Seen.count(Loc))
-        return;
-      Seen.insert(Loc);
-      auto *Scope = dyn_cast<DIScope>(Loc->getScope());
-      while (Scope) {
-        if (auto SubP = dyn_cast<DISubprogram>(Scope)) {
-          Set.insert(SubP);
-          break;
-        }
-        Scope = Scope->getScope().resolve(EmptyMap);
-      }
+      if (DISubprogram *SP = GetSubprogramForLoc(Loc))
+        Set.insert(SP);
       Loc = Loc->getInlinedAt();
     }
   }
 
-  SetVector<DISubprogram *> &
-  GetSubprogramsForFunction(Function *F, DebugInfoFinder &DbgFinder) {
-    auto It = SubprogramsForFunction.find(F);
-    if (It != SubprogramsForFunction.end())
+  DISubprogram *GetFuncSubprogram(Function *F, DebugInfoFinder &DbgFinder) {
+    auto It = FuncToSubprogram.find(F);
+    if (It != FuncToSubprogram.end())
       return It->second;
-
-    SetVector<DISubprogram *> &Ret = SubprogramsForFunction[F];
+    DISubprogram *Result = nullptr;
     for (DISubprogram *SP : DbgFinder.subprograms()) {
       if (SP->getFunction() == F) {
-        Ret.insert(SP);
+        Result = SP;
         break;
       }
     }
-
-    for (BasicBlock &BB : *F)
-      for (Instruction &I : BB)
-        CollectSubprograms(I.getDebugLoc(), Ret);
-    return Ret;
+    FuncToSubprogram[F] = Result;
+    return Result;
   }
 };
 
@@ -6771,8 +6773,18 @@ static void PatchDebugInfo(GVDebugInfoPatchCache &Cache,
 
   DIBuilder DIB(*GV->getParent());
 
-  SetVector<DISubprogram *> &Subprograms =
-      Cache.GetSubprogramsForFunction(F, DbgFinder);
+  // Only collect subprograms relevant to this GV to avoid creating
+  // O(subprograms × globals) debug instructions.
+  SetVector<DISubprogram *> Subprograms;
+
+  if (DISubprogram *SP = Cache.GetFuncSubprogram(F, DbgFinder))
+    Subprograms.insert(SP);
+
+  for (User *U : AI->users()) {
+    if (Instruction *I = dyn_cast<Instruction>(U))
+      Cache.CollectSubprograms(I->getDebugLoc(), Subprograms);
+  }
+
   for (DISubprogram *Subprogram : Subprograms) {
     DIScope *Scope = Subprogram;
     DebugLoc Loc = DebugLoc::get(DGV->getLine(), 0, Scope);