Provide reason messages for unviable inlining

[yrouban]

Summary

InlineCost's isInlineViable() is changed to return InlineResult instead of bool. This provides messages for failure reasons.
This allows to get more specific messages for cases where callsites are not viable for inlining.

[xbolva00]

lib/Analysis/InlineCost.cpp

2028

getNever should take a string

[yrouban]

lib/Analysis/InlineCost.cpp

2028

InlineResult is implicitly converted to char* Reason.
Do you propose explicit IsViable.message?

[xbolva00]

lib/Analysis/InlineCost.cpp

2028

Yes, readability would be better

[anemet]

This is great, please add some tests or check for remarks in existing tests (e.g. for the recursive case).

[yrouban]

Addressed comments:

• added a simple test
• changed implicit conversion to char* to explicit field access IsViable.message

[yrouban]

@anemet, @xbolva00, please take a look once again.

[xbolva00]

Looks fine for me

include/llvm/Analysis/InlineCost.h

 InlineCost
 getInlineCost(CallSite CS, Function *Callee, const InlineParams &Params,
               TargetTransformInfo &CalleeTTI,
               std::function<AssumptionCache &(Function &)> &GetAssumptionCache,
               Optional<function_ref<BlockFrequencyInfo &(Function &)>> GetBFI,
               ProfileSummaryInfo *PSI, OptimizationRemarkEmitter *ORE);
 
 /// Minimal filter to detect invalid constructs for inlining.
-bool isInlineViable(Function &Callee);
+InlineResult isInlineViable(Function &Callee);
 }
 
 #endif

lib/Analysis/InlineCost.cpp

if (CS.isByValArgument(I)) {
       if (PTy->getAddressSpace() != AllocaAS)
         return llvm::InlineCost::getNever("byval arguments without alloca"
                                           " address space");
     }
 
   // Calls to functions with always-inline attributes should be inlined
   // whenever possible.
   if (CS.hasFnAttr(Attribute::AlwaysInline)) {
-    if (isInlineViable(*Callee))
+    auto IsViable = isInlineViable(*Callee);
+    if (IsViable)
       return llvm::InlineCost::getAlways("always inline attribute");
-    return llvm::InlineCost::getNever("inapplicable always inline attribute");
+    return llvm::InlineCost::getNever(IsViable);
   }
 
   // Never inline functions with conflicting attributes (unless callee has
   // always-inline attribute).
   Function *Caller = CS.getCaller();
   if (!functionsHaveCompatibleAttributes(Caller, Callee, CalleeTTI))
     return llvm::InlineCost::getNever("conflicting attributes");
 
InlineCost llvm::getInlineCost(
   if (!ShouldInline && CA.getCost() < CA.getThreshold())
     return InlineCost::getNever(ShouldInline.message);
   if (ShouldInline && CA.getCost() >= CA.getThreshold())
     return InlineCost::getAlways("empty function");
 
   return llvm::InlineCost::get(CA.getCost(), CA.getThreshold());
 }
 
-bool llvm::isInlineViable(Function &F) {
+InlineResult llvm::isInlineViable(Function &F) {
   bool ReturnsTwice = F.hasFnAttribute(Attribute::ReturnsTwice);
   for (Function::iterator BI = F.begin(), BE = F.end(); BI != BE; ++BI) {
     // Disallow inlining of functions which contain indirect branches or
     // blockaddresses.
-    if (isa<IndirectBrInst>(BI->getTerminator()) || BI->hasAddressTaken())
-      return false;
+    if (isa<IndirectBrInst>(BI->getTerminator()))
+      return "contains indirect branches";
+
+    if (BI->hasAddressTaken())
+      return "uses block address";
 
     for (auto &II : *BI) {
       CallSite CS(&II);
       if (!CS)
         continue;
 
       // Disallow recursive calls.
       if (&F == CS.getCalledFunction())
-        return false;
+        return "recursive call";
 
       // Disallow calls which expose returns-twice to a function not previously
       // attributed as such.
       if (!ReturnsTwice && CS.isCall() &&
           cast<CallInst>(CS.getInstruction())->canReturnTwice())
-        return false;
+        return "exposes returns-twice attribute";
 
       if (CS.getCalledFunction())
         switch (CS.getCalledFunction()->getIntrinsicID()) {
         default:
           break;
         // Disallow inlining of @llvm.icall.branch.funnel because current
         // backend can't separate call targets from call arguments.
         case llvm::Intrinsic::icall_branch_funnel:
+          return "disallowed inlining of @llvm.icall.branch.funnel";
         // Disallow inlining functions that call @llvm.localescape. Doing this
         // correctly would require major changes to the inliner.
         case llvm::Intrinsic::localescape:
+          return "disallowed inlining of @llvm.localescape";
         // Disallow inlining of functions that initialize VarArgs with va_start.
         case llvm::Intrinsic::vastart:
-          return false;
+          return "contains VarArgs initialized with va_start";
         }
     }
   }
 
   return true;
 }
 
 // APIs to create InlineParams based on command line flags and/or other

lib/Target/AMDGPU/AMDGPUInline.cpp

InlineCost AMDGPUInliner::getInlineCost(CallSite CS) {
 
   if (CS.isNoInline())
     return llvm::InlineCost::getNever("noinline");
 
   if (!TTI.areInlineCompatible(Caller, Callee))
     return llvm::InlineCost::getNever("incompatible");
 
   if (CS.hasFnAttr(Attribute::AlwaysInline)) {
-    if (isInlineViable(*Callee))
+    auto IsViable = isInlineViable(*Callee);
+    if (IsViable)
       return llvm::InlineCost::getAlways("alwaysinline viable");
-    return llvm::InlineCost::getNever("alwaysinline unviable");
+    return llvm::InlineCost::getNever(IsViable);
   }
 
   if (isWrapperOnlyCall(CS))
     return llvm::InlineCost::getAlways("wrapper-only call");
 
   InlineParams LocalParams = Params;
   LocalParams.DefaultThreshold = (int)getInlineThreshold(CS);
   bool RemarksEnabled = false;

lib/Transforms/IPO/AlwaysInliner.cpp

 /// Note, it would be possible to go to some lengths to cache the information
 /// computed here, but as we only expect to do this for relatively few and
 /// small functions which have the explicit attribute to force inlining, it is
 /// likely not worth it in practice.
 InlineCost AlwaysInlinerLegacyPass::getInlineCost(CallSite CS) {
   Function *Callee = CS.getCalledFunction();
 
   // Only inline direct calls to functions with always-inline attributes
-  // that are viable for inlining. FIXME: We shouldn't even get here for
-  // declarations.
-  if (Callee && !Callee->isDeclaration() &&
-      CS.hasFnAttr(Attribute::AlwaysInline) && isInlineViable(*Callee))
-    return InlineCost::getAlways("always inliner");
+  // that are viable for inlining.
+  if (!Callee)
+    return InlineCost::getNever("indirect call");
+
+  // FIXME: We shouldn't even get here for declarations.
+  if (Callee->isDeclaration())
+    return InlineCost::getNever("no definition");
+
+  if (!CS.hasFnAttr(Attribute::AlwaysInline))
+    return InlineCost::getNever("no alwaysinline attribute");
+
+  auto IsViable = isInlineViable(*Callee);
+  if (!IsViable)
+    return InlineCost::getNever(IsViable);
 
-  return InlineCost::getNever("always inliner");
+  return InlineCost::getAlways("always inliner");
 }