[llvm] [WPD]: Enable speculative devirtualizatoin. (PR #159048)
via llvm-commits
llvm-commits at lists.llvm.org
Tue Sep 16 04:49:51 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms
Author: Hassnaa Hamdi (hassnaaHamdi)
<details>
<summary>Changes</summary>
- Add cl::opt 'devirtualize-speculatively' to enable it.
- Flag is disabled by default.
- It works regardless of the visibility of the object.
---
Full diff: https://github.com/llvm/llvm-project/pull/159048.diff
3 Files Affected:
- (modified) llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp (+49-15)
- (added) llvm/test/Transforms/WholeProgramDevirt/speculative-devirt-single-impl.ll (+131)
- (modified) llvm/test/Transforms/WholeProgramDevirt/virtual-const-prop-check.ll (+7)
``````````diff
diff --git a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
index 09bffa7bf5846..64f574be8fd0e 100644
--- a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
+++ b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
@@ -24,7 +24,8 @@
// returns 0, or a single vtable's function returns 1, replace each virtual
// call with a comparison of the vptr against that vtable's address.
//
-// This pass is intended to be used during the regular and thin LTO pipelines:
+// This pass is intended to be used during the regular/thin and non-LTO
+// pipelines:
//
// During regular LTO, the pass determines the best optimization for each
// virtual call and applies the resolutions directly to virtual calls that are
@@ -48,6 +49,14 @@
// is supported.
// - Import phase: (same as with hybrid case above).
//
+// During Speculative devirtualization mode -not restricted to LTO-:
+// - The pass applies speculative devirtualization without requiring any type of
+// visibility.
+// - Skips other features like virtual constant propagation, uniform return
+// value optimization, unique return value optimization and branch funnels as
+// they need LTO.
+// - This mode is enabled via 'devirtualize-speculatively' flag.
+//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/IPO/WholeProgramDevirt.h"
@@ -61,7 +70,9 @@
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
+#include "llvm/Analysis/ModuleSummaryAnalysis.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
+#include "llvm/Analysis/ProfileSummaryInfo.h"
#include "llvm/Analysis/TypeMetadataUtils.h"
#include "llvm/Bitcode/BitcodeReader.h"
#include "llvm/Bitcode/BitcodeWriter.h"
@@ -120,6 +131,13 @@ STATISTIC(NumVirtConstProp1Bit,
"Number of 1 bit virtual constant propagations");
STATISTIC(NumVirtConstProp, "Number of virtual constant propagations");
+// TODO: This option eventually should support any public visibility vtables
+// with/out LTO.
+static cl::opt<bool> ClDevirtualizeSpeculatively(
+ "devirtualize-speculatively",
+ cl::desc("Enable speculative devirtualization optimization"),
+ cl::init(false));
+
static cl::opt<PassSummaryAction> ClSummaryAction(
"wholeprogramdevirt-summary-action",
cl::desc("What to do with the summary when running this pass"),
@@ -1083,10 +1101,10 @@ bool DevirtModule::tryFindVirtualCallTargets(
if (!TM.Bits->GV->isConstant())
return false;
- // We cannot perform whole program devirtualization analysis on a vtable
- // with public LTO visibility.
- if (TM.Bits->GV->getVCallVisibility() ==
- GlobalObject::VCallVisibilityPublic)
+ // Without ClDevirtualizeSpeculatively, we cannot perform whole program
+ // devirtualization analysis on a vtable with public LTO visibility.
+ if (!ClDevirtualizeSpeculatively && TM.Bits->GV->getVCallVisibility() ==
+ GlobalObject::VCallVisibilityPublic)
return false;
Function *Fn = nullptr;
@@ -1105,6 +1123,12 @@ bool DevirtModule::tryFindVirtualCallTargets(
if (Fn->getName() == "__cxa_pure_virtual")
continue;
+ // In most cases empty functions will be overridden by the
+ // implementation of the derived class, so we can skip them.
+ if (ClDevirtualizeSpeculatively && Fn->getReturnType()->isVoidTy() &&
+ Fn->getInstructionCount() <= 1)
+ continue;
+
// We can disregard unreachable functions as possible call targets, as
// unreachable functions shouldn't be called.
if (mustBeUnreachableFunction(Fn, ExportSummary))
@@ -1223,10 +1247,12 @@ void DevirtModule::applySingleImplDevirt(VTableSlotInfo &SlotInfo,
CallTrap->setDebugLoc(CB.getDebugLoc());
}
- // If fallback checking is enabled, add support to compare the virtual
- // function pointer to the devirtualized target. In case of a mismatch,
- // fall back to indirect call.
- if (DevirtCheckMode == WPDCheckMode::Fallback) {
+ // If fallback checking or speculative devirtualization are enabled,
+ // add support to compare the virtual function pointer to the
+ // devirtualized target. In case of a mismatch, fall back to indirect
+ // call.
+ if (DevirtCheckMode == WPDCheckMode::Fallback ||
+ ClDevirtualizeSpeculatively) {
MDNode *Weights = MDBuilder(M.getContext()).createLikelyBranchWeights();
// Version the indirect call site. If the called value is equal to the
// given callee, 'NewInst' will be executed, otherwise the original call
@@ -1325,10 +1351,10 @@ bool DevirtModule::trySingleImplDevirt(
if (!IsExported)
return false;
- // If the only implementation has local linkage, we must promote to external
- // to make it visible to thin LTO objects. We can only get here during the
- // ThinLTO export phase.
- if (TheFn->hasLocalLinkage()) {
+ // Out of speculative devirtualization mode, if the only implementation has
+ // local linkage, we must promote to external to make it visible to thin LTO
+ // objects.
+ if (!ClDevirtualizeSpeculatively && TheFn->hasLocalLinkage()) {
std::string NewName = (TheFn->getName() + ".llvm.merged").str();
// Since we are renaming the function, any comdats with the same name must
@@ -2350,6 +2376,11 @@ bool DevirtModule::run() {
Function *TypeTestFunc =
Intrinsic::getDeclarationIfExists(&M, Intrinsic::type_test);
+ // If we are in speculative devirtualization mode, we can work on the public
+ // type test intrinsics.
+ if (!TypeTestFunc && ClDevirtualizeSpeculatively)
+ TypeTestFunc =
+ Intrinsic::getDeclarationIfExists(&M, Intrinsic::public_type_test);
Function *TypeCheckedLoadFunc =
Intrinsic::getDeclarationIfExists(&M, Intrinsic::type_checked_load);
Function *TypeCheckedLoadRelativeFunc = Intrinsic::getDeclarationIfExists(
@@ -2472,8 +2503,11 @@ bool DevirtModule::run() {
.WPDRes[S.first.ByteOffset];
if (tryFindVirtualCallTargets(TargetsForSlot, TypeMemberInfos,
S.first.ByteOffset, ExportSummary)) {
-
- if (!trySingleImplDevirt(ExportSummary, TargetsForSlot, S.second, Res)) {
+ bool SingleImplDevirt =
+ trySingleImplDevirt(ExportSummary, TargetsForSlot, S.second, Res);
+ // Out of speculative devirtualization mode, Try to apply virtual constant
+ // propagation or branch funneling.
+ if (!SingleImplDevirt && !ClDevirtualizeSpeculatively) {
DidVirtualConstProp |=
tryVirtualConstProp(TargetsForSlot, S.second, Res, S.first);
diff --git a/llvm/test/Transforms/WholeProgramDevirt/speculative-devirt-single-impl.ll b/llvm/test/Transforms/WholeProgramDevirt/speculative-devirt-single-impl.ll
new file mode 100644
index 0000000000000..b7d1bda8d133b
--- /dev/null
+++ b/llvm/test/Transforms/WholeProgramDevirt/speculative-devirt-single-impl.ll
@@ -0,0 +1,131 @@
+; -stats requires asserts
+; REQUIRES: asserts
+
+; Check that we can still devirtualize outside LTO mode when speculative devirtualization is enabled.
+; Check that we skip devirtualization for empty functions in speculative devirtualization mode
+
+; RUN: opt -S -passes=wholeprogramdevirt -devirtualize-speculatively \
+; RUN: -pass-remarks=wholeprogramdevirt -stats %s 2>&1 | FileCheck %s
+
+target datalayout = "e-p:64:64"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK: remark: devirt-single.cc:30:32: single-impl: devirtualized a call to vf
+; CHECK: remark: devirt-single.cc:41:32: single-impl: devirtualized a call to vf
+; CHECK: remark: devirt-single.cc:51:32: single-impl: devirtualized a call to vf
+; CHECK: remark: devirt-single.cc:13:0: devirtualized vf
+; CHECK-NOT: devirtualized
+
+ at vt1 = constant [1 x ptr] [ptr @vf], !type !8
+ at vt2 = constant [1 x ptr] [ptr @vf_empty], !type !12
+
+define i1 @vf(ptr %this) #0 !dbg !7 {
+ ret i1 true
+}
+
+; This should NOT be devirtualized because during non-lto empty functions
+; are skipped.
+define void @vf_empty(ptr %this) !dbg !11 {
+ ret void
+}
+
+; CHECK: define void @call
+define void @call(ptr %obj) #1 !dbg !5 {
+ %vtable = load ptr, ptr %obj
+ %p = call i1 @llvm.type.test(ptr %vtable, metadata !"typeid")
+ call void @llvm.assume(i1 %p)
+ %fptr = load ptr, ptr %vtable
+ ; CHECK: if.true.direct_targ:
+ ; CHECK: call i1 @vf(
+ ; CHECK: if.false.orig_indirect:
+ ; CHECK: call i1 %fptr(
+ call i1 %fptr(ptr %obj), !dbg !6
+ ret void
+}
+
+
+; CHECK: define void @call1
+define void @call1(ptr %obj) #1 !dbg !9 {
+ %vtable = load ptr, ptr %obj
+ %p = call i1 @llvm.type.test(ptr %vtable, metadata !"typeid1")
+ call void @llvm.assume(i1 %p)
+ %fptr = load ptr, ptr %vtable, align 8
+ ; CHECK: call i1 %fptr
+ %1 = call i1 %fptr(ptr %obj), !dbg !10
+ ret void
+}
+declare ptr @llvm.load.relative.i32(ptr, i32)
+
+ at vt3 = private unnamed_addr constant [1 x i32] [
+ i32 trunc (i64 sub (i64 ptrtoint (ptr dso_local_equivalent @vf to i64), i64 ptrtoint (ptr @vt3 to i64)) to i32)
+], align 4, !type !15
+
+; CHECK: define void @call2
+define void @call2(ptr %obj) #1 !dbg !13 {
+ %vtable = load ptr, ptr %obj
+ %p = call i1 @llvm.type.test(ptr %vtable, metadata !"typeid2")
+ call void @llvm.assume(i1 %p)
+ %fptr = call ptr @llvm.load.relative.i32(ptr %vtable, i32 0)
+ ; CHECK: if.true.direct_targ:
+ ; CHECK: call i1 @vf(
+ ; CHECK: if.false.orig_indirect:
+ ; CHECK: call i1 %fptr(
+ call i1 %fptr(ptr %obj), !dbg !14
+ ret void
+}
+
+ at _ZTV1A.local = private unnamed_addr constant { [3 x i32] } { [3 x i32] [
+ i32 0, ; offset to top
+ i32 0, ; rtti
+ i32 trunc (i64 sub (i64 ptrtoint (ptr dso_local_equivalent @vf to i64), i64 ptrtoint (ptr getelementptr inbounds ({ [3 x i32] }, ptr @_ZTV1A.local, i32 0, i32 0, i32 2) to i64)) to i32) ; vf_emptyunc offset
+] }, align 4, !type !18
+
+; CHECK: define void @call3
+define void @call3(ptr %obj) #1 !dbg !16 {
+ %vtable = load ptr, ptr %obj
+ %p = call i1 @llvm.type.test(ptr %vtable, metadata !"typeid3")
+ call void @llvm.assume(i1 %p)
+ %fptr = call ptr @llvm.load.relative.i32(ptr %vtable, i32 8)
+ ; CHECK: if.true.direct_targ:
+ ; CHECK: call i1 @vf(
+ ; CHECK: if.false.orig_indirect:
+ ; CHECK: call i1 %fptr(
+ call i1 %fptr(ptr %obj), !dbg !17
+ ret void
+}
+
+
+declare i1 @llvm.type.test(ptr, metadata)
+declare void @llvm.assume(i1)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3}
+!llvm.ident = !{!4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 4.0.0 (trunk 278098)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "devirt-single.cc", directory: ".")
+!2 = !{i32 2, !"Dwarf Version", i32 4}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !{!"clang version 4.0.0 (trunk 278098)"}
+!5 = distinct !DISubprogram(name: "call", linkageName: "_Z4callPv", scope: !1, file: !1, line: 29, isLocal: false, isDefinition: true, scopeLine: 9, flags: DIFlagPrototyped, isOptimized: false, unit: !0)
+!6 = !DILocation(line: 30, column: 32, scope: !5)
+!7 = distinct !DISubprogram(name: "vf", linkageName: "_ZN3vt12vfEv", scope: !1, file: !1, line: 13, isLocal: false, isDefinition: true, scopeLine: 13, flags: DIFlagPrototyped, isOptimized: false, unit: !0)
+!8 = !{i32 0, !"typeid"}
+
+!9 = distinct !DISubprogram(name: "call1", linkageName: "_Z5call1Pv", scope: !1, file: !1, line: 31, isLocal: false, isDefinition: true, scopeLine: 9, flags: DIFlagPrototyped, isOptimized: false, unit: !0)
+!10 = !DILocation(line: 35, column: 32, scope: !9)
+!11 = distinct !DISubprogram(name: "vf_empty", linkageName: "_ZN3vt18vf_emptyEv", scope: !1, file: !1, line: 23, isLocal: false, isDefinition: true, scopeLine: 23, flags: DIFlagPrototyped, isOptimized: false, unit: !0)
+!12 = !{i32 0, !"typeid1"}
+
+!13 = distinct !DISubprogram(name: "call2", linkageName: "_Z5call2Pv", scope: !1, file: !1, line: 40, isLocal: false, isDefinition: true, scopeLine: 9, flags: DIFlagPrototyped, isOptimized: false, unit: !0)
+!14 = !DILocation(line: 41, column: 32, scope: !13)
+!15 = !{i32 0, !"typeid2"}
+
+!16 = distinct !DISubprogram(name: "call3", linkageName: "_Z5call3Pv", scope: !1, file: !1, line: 50, isLocal: false, isDefinition: true, scopeLine: 9, flags: DIFlagPrototyped, isOptimized: false, unit: !0)
+!17 = !DILocation(line: 51, column: 32, scope: !16)
+!18 = !{i32 0, !"typeid3"}
+
+
+
+; CHECK: 1 wholeprogramdevirt - Number of whole program devirtualization targets
+; CHECK: 3 wholeprogramdevirt - Number of single implementation devirtualizations
diff --git a/llvm/test/Transforms/WholeProgramDevirt/virtual-const-prop-check.ll b/llvm/test/Transforms/WholeProgramDevirt/virtual-const-prop-check.ll
index d8f5c912e9a50..8327e1cfdf1d2 100644
--- a/llvm/test/Transforms/WholeProgramDevirt/virtual-const-prop-check.ll
+++ b/llvm/test/Transforms/WholeProgramDevirt/virtual-const-prop-check.ll
@@ -11,6 +11,9 @@
; Check wildcard
; RUN: opt -S -passes=wholeprogramdevirt -whole-program-visibility -pass-remarks=wholeprogramdevirt -wholeprogramdevirt-skip=vf?i1 %s 2>&1 | FileCheck %s --check-prefix=SKIP
+; Check that no stats are reported in speculative devirtualization mode as the virtual const prop is disabled.
+; RUN: opt -S -passes=wholeprogramdevirt -devirtualize-speculatively -stats %s 2>&1 | FileCheck %s --check-prefix=CHECK-SPECULATIVE-WPD
+
target datalayout = "e-p:64:64"
target triple = "x86_64-unknown-linux-gnu"
@@ -225,3 +228,7 @@ declare ptr @llvm.load.relative.i32(ptr, i32)
; CHECK: 2 wholeprogramdevirt - Number of unique return value optimizations
; CHECK: 2 wholeprogramdevirt - Number of virtual constant propagations
; CHECK: 2 wholeprogramdevirt - Number of 1 bit virtual constant propagations
+
+; CHECK-SPECULATIVE-WPD-NOT: 0 wholeprogramdevirt - Number of unique return value optimizations
+; CHECK-SPECULATIVE-WPD-NOT: 0 wholeprogramdevirt - Number of virtual constant propagations
+; CHECK-SPECULATIVE-WPD-NOT: 0 wholeprogramdevirt - Number of 1 bit virtual constant propagations
``````````
</details>
https://github.com/llvm/llvm-project/pull/159048
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