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[GVN] Improve analysis for missed optimization remark
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This change tries to handle multiple dominating users of the pointer operand
by choosing the most immediately dominating one, if possible.  While making
this change I also found that the previous implementation had a missing break
statement, making all loads with an odd number of dominating users emit an
OtherAccess value, so that has also been fixed.

Patch by Henrik G Olsson!

Differential Revision: https://reviews.llvm.org/D79097
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@anemet
anemet committed May 18, 2021
1 parent 15d4ed6 commit ab1f6ff
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Showing 3 changed files with 388 additions and 7 deletions.
54 changes: 47 additions & 7 deletions llvm/lib/Transforms/Scalar/GVN.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -931,6 +931,17 @@ static bool isLifetimeStart(const Instruction *Inst) {
return false;
}

/// Assuming To can be reached from both From and Between, does Between lie on
/// every path from From to To?
static bool liesBetween(const Instruction *From, Instruction *Between,
const Instruction *To, DominatorTree *DT) {
if (From->getParent() == Between->getParent())
return DT->dominates(From, Between);
SmallSet<BasicBlock *, 1> Exclusion;
Exclusion.insert(Between->getParent());
return !isPotentiallyReachable(From, To, &Exclusion, DT);
}

/// Try to locate the three instruction involved in a missed
/// load-elimination case that is due to an intervening store.
static void reportMayClobberedLoad(LoadInst *Load, MemDepResult DepInfo,
Expand All @@ -944,17 +955,46 @@ static void reportMayClobberedLoad(LoadInst *Load, MemDepResult DepInfo,
R << "load of type " << NV("Type", Load->getType()) << " not eliminated"
<< setExtraArgs();

for (auto *U : Load->getPointerOperand()->users())
for (auto *U : Load->getPointerOperand()->users()) {
if (U != Load && (isa<LoadInst>(U) || isa<StoreInst>(U)) &&
cast<Instruction>(U)->getFunction() == Load->getFunction() &&
DT->dominates(cast<Instruction>(U), Load)) {
// FIXME: for now give up if there are multiple memory accesses that
// dominate the load. We need further analysis to decide which one is
// that we're forwarding from.
if (OtherAccess)
OtherAccess = nullptr;
else
// Use the most immediately dominating value
if (OtherAccess) {
if (DT->dominates(cast<Instruction>(OtherAccess), cast<Instruction>(U)))
OtherAccess = U;
else
assert(DT->dominates(cast<Instruction>(U),
cast<Instruction>(OtherAccess)));
} else
OtherAccess = U;
}
}

if (!OtherAccess) {
// There is no dominating use, check if we can find a closest non-dominating
// use that lies between any other potentially available use and Load.
for (auto *U : Load->getPointerOperand()->users()) {
if (U != Load && (isa<LoadInst>(U) || isa<StoreInst>(U)) &&
cast<Instruction>(U)->getFunction() == Load->getFunction() &&
isPotentiallyReachable(cast<Instruction>(U), Load, nullptr, DT)) {
if (OtherAccess) {
if (liesBetween(cast<Instruction>(OtherAccess), cast<Instruction>(U),
Load, DT)) {
OtherAccess = U;
} else if (!liesBetween(cast<Instruction>(U),
cast<Instruction>(OtherAccess), Load, DT)) {
// These uses are both partially available at Load were it not for
// the clobber, but neither lies strictly after the other.
OtherAccess = nullptr;
break;
} // else: keep current OtherAccess since it lies between U and Load
} else {
OtherAccess = U;
}
}
}
}

if (OtherAccess)
R << " in favor of " << NV("OtherAccess", OtherAccess);
Expand Down
136 changes: 136 additions & 0 deletions llvm/test/Transforms/GVN/opt-remarks-multiple-users.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,136 @@
; RUN: opt < %s -gvn -o /dev/null -pass-remarks-output=%t -S
; RUN: cat %t | FileCheck %s

; CHECK: --- !Missed
; CHECK-NEXT: Pass: gvn
; CHECK-NEXT: Name: LoadClobbered
; CHECK-NEXT: DebugLoc: { File: '/tmp/s.c', Line: 2, Column: 2 }
; CHECK-NEXT: Function: multipleUsers
; CHECK-NEXT: Args:
; CHECK-NEXT: - String: 'load of type '
; CHECK-NEXT: - Type: i32
; CHECK-NEXT: - String: ' not eliminated'
; CHECK-NEXT: - String: ' in favor of '
; CHECK-NEXT: - OtherAccess: store
; CHECK-NEXT: - String: ' because it is clobbered by '
; CHECK-NEXT: - ClobberedBy: call
; CHECK-NEXT: DebugLoc: { File: '/tmp/s.c', Line: 1, Column: 1 }
; CHECK-NEXT: ...
; CHECK: --- !Missed
; CHECK-NEXT: Pass: gvn
; CHECK-NEXT: Name: LoadClobbered
; CHECK-NEXT: DebugLoc: { File: '/tmp/s.c', Line: 4, Column: 4 }
; CHECK-NEXT: Function: multipleUsers
; CHECK-NEXT: Args:
; CHECK-NEXT: - String: 'load of type '
; CHECK-NEXT: - Type: i32
; CHECK-NEXT: - String: ' not eliminated'
; CHECK-NEXT: - String: ' in favor of '
; CHECK-NEXT: - OtherAccess: load
; CHECK-NEXT: DebugLoc: { File: '/tmp/s.c', Line: 2, Column: 2 }
; CHECK-NEXT: - String: ' because it is clobbered by '
; CHECK-NEXT: - ClobberedBy: call
; CHECK-NEXT: DebugLoc: { File: '/tmp/s.c', Line: 3, Column: 3 }
; CHECK-NEXT: ...

; ModuleID = 'bugpoint-reduced-simplified.bc'
source_filename = "gvn-test.c"
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"

; Tests that a last clobbering use can be determined even in the presence of
; multiple users, given that one of them lies on a path between every other
; potentially clobbering use and the load.

define dso_local void @multipleUsers(i32* %a, i32 %b) local_unnamed_addr #0 {
entry:
store i32 %b, i32* %a, align 4
tail call void @clobberingFunc() #1, !dbg !10
%0 = load i32, i32* %a, align 4, !dbg !11
tail call void @clobberingFunc() #1, !dbg !12
%1 = load i32, i32* %a, align 4, !dbg !13
%add2 = add nsw i32 %1, %0
ret void
}

; CHECK: --- !Missed
; CHECK-NEXT: Pass: gvn
; CHECK-NEXT: Name: LoadClobbered
; CHECK-NEXT: DebugLoc: { File: '/tmp/s.c', Line: 2, Column: 2 }
; CHECK-NEXT: Function: multipleUsers2
; CHECK-NEXT: Args:
; CHECK-NEXT: - String: 'load of type '
; CHECK-NEXT: - Type: i32
; CHECK-NEXT: - String: ' not eliminated'
; CHECK-NEXT: - String: ' in favor of '
; CHECK-NEXT: - OtherAccess: store
; CHECK-NEXT: - String: ' because it is clobbered by '
; CHECK-NEXT: - ClobberedBy: call
; CHECK-NEXT: DebugLoc: { File: '/tmp/s.c', Line: 1, Column: 1 }
; CHECK-NEXT: ...
; CHECK: --- !Missed
; CHECK-NEXT: Pass: gvn
; CHECK-NEXT: Name: LoadClobbered
; CHECK-NEXT: DebugLoc: { File: '/tmp/s.c', Line: 4, Column: 4 }
; CHECK-NEXT: Function: multipleUsers2
; CHECK-NEXT: Args:
; CHECK-NEXT: - String: 'load of type '
; CHECK-NEXT: - Type: i32
; CHECK-NEXT: - String: ' not eliminated'
; CHECK-NEXT: - String: ' in favor of '
; CHECK-NEXT: - OtherAccess: load
; CHECK-NEXT: DebugLoc: { File: '/tmp/s.c', Line: 2, Column: 2 }
; CHECK-NEXT: - String: ' because it is clobbered by '
; CHECK-NEXT: - ClobberedBy: call
; CHECK-NEXT: DebugLoc: { File: '/tmp/s.c', Line: 3, Column: 3 }
; CHECK-NEXT: ...

; Ignore uses in other functions

define dso_local void @multipleUsers2(i32 %b) local_unnamed_addr #0 {
entry:
store i32 %b, i32* @g, align 4
tail call void @clobberingFunc() #1, !dbg !15
%0 = load i32, i32* @g, align 4, !dbg !16
tail call void @clobberingFunc() #1, !dbg !17
%1 = load i32, i32* @g, align 4, !dbg !18
%add3 = add nsw i32 %1, %0
ret void
}

declare dso_local void @clobberingFunc() local_unnamed_addr #0

@g = external global i32

define dso_local void @globalUser(i32 %b) local_unnamed_addr #0 {
entry:
store i32 %b, i32* @g, align 4
ret void
}


attributes #0 = { "use-soft-float"="false" }
attributes #1 = { nounwind }

!llvm.dbg.cu = !{!1}
!llvm.module.flags = !{!4, !5, !6}
!llvm.ident = !{!0}

!0 = !{!"clang version 10.0.0 (git@github.com:llvm/llvm-project.git a2f6ae9abffcba260c22bb235879f0576bf3b783)"}
!1 = distinct !DICompileUnit(language: DW_LANG_C99, file: !2, producer: "clang version 4.0.0 (trunk 282540) (llvm/trunk 282542)", isOptimized: true, runtimeVersion: 0, emissionKind: LineTablesOnly, enums: !3)
!2 = !DIFile(filename: "/tmp/s.c", directory: "/tmp")
!3 = !{}
!4 = !{i32 2, !"Dwarf Version", i32 4}
!5 = !{i32 2, !"Debug Info Version", i32 3}
!6 = !{i32 1, !"PIC Level", i32 2}
!8 = distinct !DISubprogram(name: "multipleUsers", scope: !2, file: !2, line: 1, type: !9, isLocal: false, isDefinition: true, scopeLine: 1, isOptimized: true, unit: !1, retainedNodes: !3)
!9 = !DISubroutineType(types: !3)
!10 = !DILocation(line: 1, column: 1, scope: !8)
!11 = !DILocation(line: 2, column: 2, scope: !8)
!12 = !DILocation(line: 3, column: 3, scope: !8)
!13 = !DILocation(line: 4, column: 4, scope: !8)
!14 = distinct !DISubprogram(name: "multipleUsers2", scope: !2, file: !2, line: 2, type: !9, isLocal: false, isDefinition: true, scopeLine: 1, isOptimized: true, unit: !1, retainedNodes: !3)
!15 = !DILocation(line: 1, column: 1, scope: !14)
!16 = !DILocation(line: 2, column: 2, scope: !14)
!17 = !DILocation(line: 3, column: 3, scope: !14)
!18 = !DILocation(line: 4, column: 4, scope: !14)
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