[WPD][ThinLTO]Add cutoff option for WPD (#113383)

This option applies for _import_ WPD (i.e., when `DevirtModule` pass
de-virtualizes according to an imported summary, in ThinLTO backend
pipeline). It's meant for debugging (e.g., bisection).

Author: mingmingl-llvm

llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp

@@ -168,6 +168,14 @@ static cl::list<std::string>
                       cl::desc("Prevent function(s) from being devirtualized"),
                       cl::Hidden, cl::CommaSeparated);
 
+/// If explicitly specified, the devirt module pass will stop transformation
+/// once the total number of devirtualizations reach the cutoff value. Setting
+/// this option to 0 explicitly will do 0 devirtualization.
+static cl::opt<unsigned> WholeProgramDevirtCutoff(
+    "wholeprogramdevirt-cutoff",
+    cl::desc("Max number of devirtualizations for devirt module pass"),
+    cl::init(0));
+
 /// Mechanism to add runtime checking of devirtualization decisions, optionally
 /// trapping or falling back to indirect call on any that are not correct.
 /// Trapping mode is useful for debugging undefined behavior leading to failures
@@ -316,6 +324,9 @@ VirtualCallTarget::VirtualCallTarget(GlobalValue *Fn, const TypeMemberInfo *TM)
 
 namespace {
 
+// Tracks the number of devirted calls in the IR transformation.
+static unsigned NumDevirtCalls = 0;
+
 // A slot in a set of virtual tables. The TypeID identifies the set of virtual
 // tables, and the ByteOffset is the offset in bytes from the address point to
 // the virtual function pointer.
@@ -1169,10 +1180,16 @@ void DevirtModule::applySingleImplDevirt(VTableSlotInfo &SlotInfo,
       if (!OptimizedCalls.insert(&VCallSite.CB).second)
         continue;
 
+      // Stop when the number of devirted calls reaches the cutoff.
+      if (WholeProgramDevirtCutoff.getNumOccurrences() > 0 &&
+          NumDevirtCalls >= WholeProgramDevirtCutoff)
+        return;
+
       if (RemarksEnabled)
         VCallSite.emitRemark("single-impl",
                              TheFn->stripPointerCasts()->getName(), OREGetter);
       NumSingleImpl++;
+      NumDevirtCalls++;
       auto &CB = VCallSite.CB;
       assert(!CB.getCalledFunction() && "devirtualizing direct call?");
       IRBuilder<> Builder(&CB);

llvm/test/Transforms/WholeProgramDevirt/import.ll

@@ -8,6 +8,12 @@
 ; RUN: opt -S -passes=wholeprogramdevirt -wholeprogramdevirt-summary-action=import -wholeprogramdevirt-read-summary=%S/Inputs/import-vcp-branch-funnel.yaml < %s | FileCheck --check-prefixes=CHECK,VCP,VCP-X86,VCP64,BRANCH-FUNNEL %s
 ; RUN: opt -S -passes=wholeprogramdevirt -wholeprogramdevirt-summary-action=import -wholeprogramdevirt-read-summary=%S/Inputs/import-branch-funnel.yaml < %s | FileCheck --check-prefixes=CHECK,BRANCH-FUNNEL,BRANCH-FUNNEL-NOVCP %s
 
+; Cutoff value is not explicitly set. Expect 3 remark messages.
+; RUN: opt -S -passes=wholeprogramdevirt -wholeprogramdevirt-summary-action=import -pass-remarks=wholeprogramdevirt -wholeprogramdevirt-read-summary=%S/Inputs/import-single-impl.yaml < %s  2>&1 | grep "single-impl" | count 3
+; Cutoff value is set to 1. Expect one remark messages.
+; RUN: opt -S -passes=wholeprogramdevirt -wholeprogramdevirt-summary-action=import -pass-remarks=wholeprogramdevirt -wholeprogramdevirt-cutoff=1  -wholeprogramdevirt-read-summary=%S/Inputs/import-single-impl.yaml < %s  2>&1 | grep "single-impl" | count 1
+; Cutoff value is explicitly set to zero. Expect no remark message.
+; RUN: opt -S -passes=wholeprogramdevirt -wholeprogramdevirt-summary-action=import -pass-remarks=wholeprogramdevirt -wholeprogramdevirt-cutoff=0  -wholeprogramdevirt-read-summary=%S/Inputs/import-single-impl.yaml < %s 2>&1  | FileCheck -implicit-check-not="remark" %s
 target datalayout = "e-p:64:64"
 target triple = "x86_64-unknown-linux-gnu"