[llvm] 9c49f8d - [LTO][WPD] Ignore unreachable function by analyzing IR.

[via llvm-commits]

Author: minglotus-6
Date: 2021-12-21T18:13:03Z
New Revision: 9c49f8d70592cd421691a194da259652fc7aec35

URL: https://github.com/llvm/llvm-project/commit/9c49f8d70592cd421691a194da259652fc7aec35
DIFF: https://github.com/llvm/llvm-project/commit/9c49f8d70592cd421691a194da259652fc7aec35.diff

LOG: [LTO][WPD] Ignore unreachable function by analyzing IR.

In regular LTO, analyze IR and discard unreachable functions when finding virtual call targets.

Differential Revision: https://reviews.llvm.org/D116056

Added: 
    llvm/test/Transforms/WholeProgramDevirt/devirt_single_after_filtering_unreachable_function.ll

Modified: 
    llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
index 18049d22c702b..6acace1d9fd43 100644
--- a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
+++ b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
@@ -678,6 +678,17 @@ struct DevirtModule {
   static ValueInfo lookUpFunctionValueInfo(Function *TheFn,
                                            ModuleSummaryIndex *ExportSummary);
 
+  // Returns true if the function definition must be unreachable.
+  //
+  // Note if this helper function returns true, `F` is guaranteed
+  // to be unreachable; if it returns false, `F` might still
+  // be unreachable but not covered by this helper function.
+  //
+  // Implementation-wise, if function definition is present, IR is analyzed; if
+  // not, look up function flags from ExportSummary as a fallback.
+  static bool mustBeUnreachableFunction(Function *const F,
+                                        ModuleSummaryIndex *ExportSummary);
+
   // Lower the module using the action and summary passed as command line
   // arguments. For testing purposes only.
   static bool
@@ -1038,10 +1049,8 @@ bool DevirtModule::tryFindVirtualCallTargets(
 
     // We can disregard unreachable functions as possible call targets, as
     // unreachable functions shouldn't be called.
-    if (ExportSummary && (mustBeUnreachableFunction(
-                             lookUpFunctionValueInfo(Fn, ExportSummary)))) {
+    if (mustBeUnreachableFunction(Fn, ExportSummary))
       continue;
-    }
 
     TargetsForSlot.push_back({Fn, &TM});
   }
@@ -2087,6 +2096,20 @@ DevirtModule::lookUpFunctionValueInfo(Function *TheFn,
   return TheFnVI;
 }
 
+bool DevirtModule::mustBeUnreachableFunction(
+    Function *const F, ModuleSummaryIndex *ExportSummary) {
+  // First, learn unreachability by analyzing function IR.
+  if (!F->isDeclaration()) {
+    // A function must be unreachable if its entry block ends with an
+    // 'unreachable'.
+    return isa<UnreachableInst>(F->getEntryBlock().getTerminator());
+  }
+  // Learn unreachability from ExportSummary if ExportSummary is present.
+  return ExportSummary &&
+         ::mustBeUnreachableFunction(
+             DevirtModule::lookUpFunctionValueInfo(F, ExportSummary));
+}
+
 bool DevirtModule::run() {
   // If only some of the modules were split, we cannot correctly perform
   // this transformation. We already checked for the presense of type tests

diff  --git a/llvm/test/Transforms/WholeProgramDevirt/devirt_single_after_filtering_unreachable_function.ll b/llvm/test/Transforms/WholeProgramDevirt/devirt_single_after_filtering_unreachable_function.ll
new file mode 100644
index 0000000000000..994a3b58e75cd
--- /dev/null
+++ b/llvm/test/Transforms/WholeProgramDevirt/devirt_single_after_filtering_unreachable_function.ll
@@ -0,0 +1,61 @@
+; Test that regular LTO will analyze IR, detect unreachable functions and discard unreachable functions
+; when finding virtual call targets.
+; In this test case, the unreachable function is the virtual deleting destructor of an abstract class.
+
+; RUN: opt -S -passes=wholeprogramdevirt -whole-program-visibility -pass-remarks=wholeprogramdevirt %s 2>&1 | FileCheck %s
+
+; CHECK: remark: tmp.cc:21:3: single-impl: devirtualized a call to _ZN7DerivedD0Ev
+; CHECK: remark: <unknown>:0:0: devirtualized _ZN7DerivedD0Ev
+
+source_filename = "tmp.cc"
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+%Derived = type { %Base }
+%Base = type { i32 (...)** }
+
+ at _ZTV7Derived = constant { [3 x i8*] } { [3 x i8*] [ i8* null, i8* null, i8* bitcast (void (%Derived*)* @_ZN7DerivedD0Ev to i8*)] }, !type !0, !type !1, !type !2, !type !3
+ at _ZTV4Base = constant { [3 x i8*] } { [3 x i8*] [ i8* null, i8* null, i8* bitcast (void (%Base*)* @_ZN4BaseD0Ev to i8*)] }, !type !0, !type !1
+
+declare i1 @llvm.type.test(i8*, metadata)
+
+declare void @llvm.assume(i1)
+
+define i32 @func(%Base* %b) {
+entry:
+  %0 = bitcast %Base* %b to void (%Base*)***, !dbg !11
+  %vtable = load void (%Base*)**, void (%Base*)*** %0, !dbg !11
+  %1 = bitcast void (%Base*)** %vtable to i8*, !dbg !11
+  %2 = tail call i1 @llvm.type.test(i8* %1, metadata !"_ZTS4Base"), !dbg !11
+  tail call void @llvm.assume(i1 %2), !dbg !11
+  %vfn = getelementptr inbounds void (%Base*)*, void (%Base*)** %vtable, i64 0, !dbg !11
+  %3 = load void (%Base*)*, void (%Base*)** %vfn, !dbg !11
+  tail call void %3(%Base* %b), !dbg !11
+  ret i32 0
+}
+
+define void @_ZN7DerivedD0Ev(%Derived* %this) {
+entry:
+  ret void
+}
+
+define void @_ZN4BaseD0Ev(%Base* %this) {
+entry:
+  tail call void @llvm.trap()
+  unreachable
+}
+
+declare void @llvm.trap()
+
+!llvm.dbg.cu = !{!5}
+!llvm.module.flags = !{!7}
+
+!0 = !{i64 16, !"_ZTS4Base"}
+!1 = !{i64 32, !"_ZTSM4BaseFvvE.virtual"}
+!2 = !{i64 16, !"_ZTS7Derived"}
+!3 = !{i64 32, !"_ZTSM7DerivedFvvE.virtual"}
+!5 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !6)
+!6 = !DIFile(filename: "tmp.cc", directory: "")
+!7 = !{i32 2, !"Debug Info Version", i32 3}
+!10= distinct !DISubprogram(name: "func", scope: !6, file: !6, unit: !5)
+!11 = !DILocation(line: 21, column: 3, scope: !10)