[WPD] Change Devirt Cutoff to use DebugCounter

[boomanaiden154]

This removes the presence of global state from within the pass which is blocking some efforts around test daemonization and is not good design practice in general for LLVM. See
https://discourse.llvm.org/t/rfc-reducing-process-creation-overhead-in-llvm-regression-tests/88612/11 for more discussion.

This patch replaces the usage of global state with a DebugCounter, which helps fix the global state problem and also increases the flexibility of the option as now an explicit range can be passed.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Aiden Grossman (boomanaiden154)

Changes

This removes the presence of global state from within the pass which is blocking some efforts around test daemonization and is not good design practice in general for LLVM. See
https://discourse.llvm.org/t/rfc-reducing-process-creation-overhead-in-llvm-regression-tests/88612/11 for more discussion.

This patch replaces the usage of global state with a DebugCounter, which helps fix the global state problem and also increases the flexibility of the option as now an explicit range can be passed.

---

Full diff: https://github.com/llvm/llvm-project/pull/170009.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp (+5-15)
• (modified) llvm/test/Transforms/WholeProgramDevirt/import.ll (+5-5)

diff --git a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
index 2dd0fde6b34d6..4642da0abdc13 100644
--- a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
+++ b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
@@ -99,6 +99,7 @@
 #include "llvm/IR/ProfDataUtils.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/CommandLine.h"
+#include "llvm/Support/DebugCounter.h"
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/FileSystem.h"
@@ -130,6 +131,8 @@ STATISTIC(NumUniqueRetVal, "Number of unique return value optimizations");
 STATISTIC(NumVirtConstProp1Bit,
           "Number of 1 bit virtual constant propagations");
 STATISTIC(NumVirtConstProp, "Number of virtual constant propagations");
+DEBUG_COUNTER(CallsToDevirt, "calls-to-devirt",
+              "Controls how many calls should be devirtualized.");
 
 namespace llvm {
 
@@ -219,14 +222,6 @@ static cl::opt<bool> WholeProgramDevirtKeepUnreachableFunction(
     cl::desc("Regard unreachable functions as possible devirtualize targets."),
     cl::Hidden, cl::init(true));
 
-/// 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
@@ -377,9 +372,6 @@ 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.
@@ -1216,15 +1208,13 @@ void DevirtModule::applySingleImplDevirt(VTableSlotInfo &SlotInfo,
         continue;
 
       // Stop when the number of devirted calls reaches the cutoff.
-      if (WholeProgramDevirtCutoff.getNumOccurrences() > 0 &&
-          NumDevirtCalls >= WholeProgramDevirtCutoff)
-        return;
+      if (!DebugCounter::shouldExecute(CallsToDevirt))
+        continue;
 
       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);
diff --git a/llvm/test/Transforms/WholeProgramDevirt/import.ll b/llvm/test/Transforms/WholeProgramDevirt/import.ll
index de25bc10a7c12..153f18e4fbc3a 100644
--- a/llvm/test/Transforms/WholeProgramDevirt/import.ll
+++ b/llvm/test/Transforms/WholeProgramDevirt/import.ll
@@ -8,12 +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.
+; Devirt calls debug counter 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
+; Devirt calls debug counter is set to 1. Expect one remark messages.
+; RUN: opt -S -passes=wholeprogramdevirt -wholeprogramdevirt-summary-action=import -pass-remarks=wholeprogramdevirt -debug-counter=calls-to-devirt=0  -wholeprogramdevirt-read-summary=%S/Inputs/import-single-impl.yaml < %s  2>&1 | grep "single-impl" | count 1
+; Devirt calls debug counter is set outside the range of calls. Expect no remark message.
+; RUN: opt -S -passes=wholeprogramdevirt -wholeprogramdevirt-summary-action=import -pass-remarks=wholeprogramdevirt -debug-counter=calls-to-devirt=9999  -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"
 

[mingmingl-llvm]

thanks for the change Aiden! LGTM with some suggestions

[mingmingl-llvm]

nit: setting -debug-counter=calls-to-devirt=2 -print-debug-counter-queries devirts the third callsite, and grep "single-impl" also gives 1.

Could you tighten up the test to add -print-debug-counter-queries and also test that the debug counter output?

-debug-counter=calls-to-devirt=0 -print-debug-counter-queries gives the following output.

DebugCounter calls-to-devirt=0 execute
remark: <unknown>:0:0: single-impl: devirtualized a call to singleimpl1
DebugCounter calls-to-devirt=1 skip
DebugCounter calls-to-devirt=2 skip

[mingmingl-llvm]

My understanding of

llvm-project/llvm/include/llvm/Support/DebugCounter.h

Lines 156 to 163 in 24b87b8

	static bool isCountingEnabled() {
	// Compile to nothing when debugging is off
	#ifdef NDEBUG
	return false;
	#else
	return instance().Enabled || instance().ShouldPrintCounter;
	#endif
	}

is that debug counter needs -DLLVM_ENABLE_ASSERTIONS=ON, and the test will need something like 'REQUIRES: asserts'.

[mingmingl-llvm]

fyi I'm planning to land https://github.com/llvm/llvm-project/pull/170011/files after this change to make the multi-line RUN statement easier to modify.