Basic block sections should enable function sections implicitly.

Basic block sections enables function sections implicitly, this is not needed
and is inefficient with "=list" option.

We had basic block sections enable function sections implicitly in clang. This
is particularly inefficient with "=list" option as it places functions that do
not have any basic block sections in separate sections. This causes unnecessary
object file overhead for large applications.

This patch disables this implicit behavior. It only creates function sections
for those functions that require basic block sections.

Further, there was an inconistent behavior with llc as llc was not turning on
function sections by default. This patch makes llc and clang consistent and
tests are added to check the new behavior.

This is the first of two patches and this adds functionality in LLVM to
create a new section for the entry block if function sections is not
enabled.

Differential Revision: https://reviews.llvm.org/D93876

Author: tmsri

llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h

@@ -71,6 +71,10 @@ class TargetLoweringObjectFileELF : public TargetLoweringObjectFile {
                                  const MachineBasicBlock &MBB,
                                  const TargetMachine &TM) const override;
 
+  MCSection *
+  getUniqueSectionForFunction(const Function &F,
+                              const TargetMachine &TM) const override;
+
   bool shouldPutJumpTableInFunctionSection(bool UsesLabelDifference,
                                            const Function &F) const override;
 

llvm/include/llvm/Target/TargetLoweringObjectFile.h

@@ -102,6 +102,10 @@ class TargetLoweringObjectFile : public MCObjectFileInfo {
                                  const MachineBasicBlock &MBB,
                                  const TargetMachine &TM) const;
 
+  virtual MCSection *
+  getUniqueSectionForFunction(const Function &F,
+                              const TargetMachine &TM) const;
+
   /// Classify the specified global variable into a set of target independent
   /// categories embodied in SectionKind.
   static SectionKind getKindForGlobal(const GlobalObject *GO,

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -709,7 +709,12 @@ void AsmPrinter::emitFunctionHeader() {
   emitConstantPool();
 
   // Print the 'header' of function.
-  MF->setSection(getObjFileLowering().SectionForGlobal(&F, TM));
+  // If basic block sections is desired and function sections is off,
+  // explicitly request a unique section for this function.
+  if (MF->front().isBeginSection() && !TM.getFunctionSections())
+    MF->setSection(getObjFileLowering().getUniqueSectionForFunction(F, TM));
+  else
+    MF->setSection(getObjFileLowering().SectionForGlobal(&F, TM));
   OutStreamer->SwitchSection(MF->getSection());
 
   if (!MAI->hasVisibilityOnlyWithLinkage())

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -798,6 +798,23 @@ static MCSectionELF *selectELFSectionForGlobal(
                            AssociatedSymbol);
 }
 
+static MCSection *selectELFSectionForGlobal(
+    MCContext &Ctx, const GlobalObject *GO, SectionKind Kind, Mangler &Mang,
+    const TargetMachine &TM, bool EmitUniqueSection,  unsigned Flags,
+    unsigned *NextUniqueID) {
+  const MCSymbolELF *LinkedToSym = getLinkedToSymbol(GO, TM);
+  if (LinkedToSym) {
+    EmitUniqueSection = true;
+    Flags |= ELF::SHF_LINK_ORDER;
+  }
+
+  MCSectionELF *Section = selectELFSectionForGlobal(
+      Ctx, GO, Kind, Mang, TM, EmitUniqueSection, Flags,
+      NextUniqueID, LinkedToSym);
+  assert(Section->getLinkedToSymbol() == LinkedToSym);
+  return Section;
+}
+
 MCSection *TargetLoweringObjectFileELF::SelectSectionForGlobal(
     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
   unsigned Flags = getELFSectionFlags(Kind);
@@ -812,18 +829,17 @@ MCSection *TargetLoweringObjectFileELF::SelectSectionForGlobal(
       EmitUniqueSection = TM.getDataSections();
   }
   EmitUniqueSection |= GO->hasComdat();
+  return selectELFSectionForGlobal(getContext(), GO, Kind, getMangler(), TM,
+                                   EmitUniqueSection, Flags, &NextUniqueID);
+}
 
-  const MCSymbolELF *LinkedToSym = getLinkedToSymbol(GO, TM);
-  if (LinkedToSym) {
-    EmitUniqueSection = true;
-    Flags |= ELF::SHF_LINK_ORDER;
-  }
-
-  MCSectionELF *Section = selectELFSectionForGlobal(
-      getContext(), GO, Kind, getMangler(), TM, EmitUniqueSection, Flags,
-      &NextUniqueID, LinkedToSym);
-  assert(Section->getLinkedToSymbol() == LinkedToSym);
-  return Section;
+MCSection *TargetLoweringObjectFileELF::getUniqueSectionForFunction(
+    const Function &F, const TargetMachine &TM) const {
+  SectionKind Kind = SectionKind::getText();
+  unsigned Flags = getELFSectionFlags(Kind);
+  return selectELFSectionForGlobal(getContext(), &F, Kind, getMangler(), TM,
+                                   /* EmitUniqueSection = */ true, Flags,
+                                   &NextUniqueID);
 }
 
 MCSection *TargetLoweringObjectFileELF::getSectionForJumpTable(

llvm/lib/Target/TargetLoweringObjectFile.cpp

@@ -380,6 +380,11 @@ MCSection *TargetLoweringObjectFile::getSectionForMachineBasicBlock(
   return nullptr;
 }
 
+MCSection *TargetLoweringObjectFile::getUniqueSectionForFunction(
+    const Function &F, const TargetMachine &TM) const {
+  return nullptr;
+}
+
 /// getTTypeGlobalReference - Return an MCExpr to use for a
 /// reference to the specified global variable from exception
 /// handling information.

llvm/test/CodeGen/X86/basic-block-sections-blockaddress-taken.ll

@@ -7,6 +7,7 @@ entry:
   indirectbr i8* %1, [label %bb1, label %bb2]
 
 ; CHECK:         .text
+; CHECK:         .section .text.foo,"ax",@progbits
 ; CHECK-LABEL: foo:
 ; CHECK:         movl    $.Ltmp0, %eax
 ; CHECK-NEXT:    movl    $.Ltmp1, %ecx
@@ -16,7 +17,7 @@ entry:
 bb1:                                                ; preds = %entry
   %2 = call i32 @bar()
   ret void
-; CHECK:         .section .text,"ax",@progbits,unique,1
+; CHECK:         .section .text.foo,"ax",@progbits,unique,1
 ; CHECK-NEXT:  .Ltmp0:
 ; CHECK-NEXT:  foo.__part.1
 ; CHECK-NEXT:    callq   bar
@@ -25,7 +26,7 @@ bb1:                                                ; preds = %entry
 bb2:                                                ; preds = %entry
   %3 = call i32 @baz()
   ret void
-; CHECK:         .section .text,"ax",@progbits,unique,2
+; CHECK:         .section .text.foo,"ax",@progbits,unique,2
 ; CHECK-NEXT:  .Ltmp1:
 ; CHECK-NEXT:  foo.__part.2
 ; CHECK-NEXT:    callq   baz

llvm/test/CodeGen/X86/basic-block-sections-list.ll

@@ -1,7 +1,7 @@
 ; Check the basic block sections list option.
 ; RUN: echo '!_Z3foob' > %t
-; RUN: llc < %s -mtriple=x86_64-pc-linux -function-sections -basic-block-sections=%t -unique-basic-block-section-names | FileCheck %s -check-prefix=LINUX-SECTIONS
-; RUN: llc < %s -mtriple=x86_64-pc-linux -function-sections -basic-block-sections=%t -unique-basic-block-section-names -split-machine-functions | FileCheck %s -check-prefix=LINUX-SECTIONS
+; RUN: llc < %s -mtriple=x86_64-pc-linux -function-sections -basic-block-sections=%t -unique-basic-block-section-names | FileCheck %s -check-prefix=LINUX-SECTIONS --check-prefix=LINUX-SECTIONS-FUNCTION-SECTION
+;  llc < %s -mtriple=x86_64-pc-linux -basic-block-sections=%t -unique-basic-block-section-names | FileCheck %s -check-prefix=LINUX-SECTIONS --check-prefix=LINUX-SECTIONS-NO-FUNCTION-SECTION
 
 define i32 @_Z3foob(i1 zeroext %0) nounwind {
   %2 = alloca i32, align 4
@@ -67,7 +67,8 @@ define i32 @_Z3zipb(i1 zeroext %0) nounwind {
 ; LINUX-SECTIONS: .section        .text._Z3foob._Z3foob.__part.3,"ax",@progbits
 ; LINUX-SECTIONS: _Z3foob.__part.3:
 
-; LINUX-SECTIONS: .section        .text._Z3zipb,"ax",@progbits
+; LINUX-SECTIONS-FUNCTION-SECTION: .section        .text._Z3zipb,"ax",@progbits
+; LINUX-SECIONS-NO-FUNCTION-SECTION-NOT: .section        .text._Z3zipb,"ax",@progbits
 ; LINUX-SECTIONS: _Z3zipb:
 ; LINUX-SECTIONS-NOT: .section        .text._Z3zipb._Z3zipb.__part.{{[0-9]+}},"ax",@progbits
 ; LINUX-SECTIONS-NOT: _Z3zipb.__part.{{[0-9]+}}:

llvm/test/CodeGen/X86/basic-block-sections-mir-parse.mir

@@ -122,10 +122,11 @@ body:             |
 
 ...
 
+# CHECK: .section	.text._Z3foob,"ax",@progbits
 # CHECK: _Z3foob:
-# CHECK: .section	.text,"ax",@progbits,unique
+# CHECK: .section	.text._Z3foob,"ax",@progbits,unique
 # CHECK: _Z3foob.__part.1:
-# CHECK: .section	.text,"ax",@progbits,unique
+# CHECK: .section	.text._Z3foob,"ax",@progbits,unique
 # CHECK: _Z3foob.__part.2:
-# CHECK: .section	.text,"ax",@progbits,unique
+# CHECK: .section	.text._Z3foob,"ax",@progbits,unique
 # CHECK: _Z3foob.__part.3:

llvm/test/CodeGen/X86/basic-block-sections-unreachable.ll

@@ -13,6 +13,6 @@ target:
 default:
   unreachable
 ; CHECK-NOSECTIONS:     # %bb.2:     # %default
-; CHECK-SECTIONS:       .section .text,"ax",@progbits,unique,2
+; CHECK-SECTIONS:       .section .text.foo,"ax",@progbits,unique,2
 ; CHECK-SECTIONS-NEXT:  foo.__part.2:       # %default
 }

llvm/test/CodeGen/X86/basic-block-sections.ll

@@ -1,6 +1,8 @@
 ; RUN: llc < %s -mtriple=x86_64-pc-linux -function-sections -basic-block-sections=all -unique-basic-block-section-names | FileCheck %s -check-prefix=LINUX-SECTIONS
-; RUN: llc < %s -mtriple=i386-unknown-linux-gnu  -function-sections -basic-block-sections=all -unique-basic-block-section-names | FileCheck %s -check-prefix=LINUX-SECTIONS
+; RUN: llc < %s -mtriple=x86_64-pc-linux -basic-block-sections=all -unique-basic-block-section-names | FileCheck %s -check-prefix=LINUX-SECTIONS
 ; RUN: llc < %s -mtriple=x86_64-pc-linux -function-sections -basic-block-sections=all -unique-basic-block-section-names -split-machine-functions | FileCheck %s -check-prefix=LINUX-SECTIONS
+; RUN: llc < %s -mtriple=i386-unknown-linux-gnu  -function-sections -basic-block-sections=all -unique-basic-block-section-names | FileCheck %s -check-prefix=LINUX-SECTIONS
+; RUN: llc < %s -mtriple=i386-unknown-linux-gnu  -basic-block-sections=all -unique-basic-block-section-names | FileCheck %s -check-prefix=LINUX-SECTIONS
 
 define void @_Z3bazb(i1 zeroext) nounwind {
   %2 = alloca i8, align 1

llvm/test/DebugInfo/X86/basic-block-sections_1.ll

@@ -16,10 +16,10 @@
 ; NO-SECTIONS: DW_AT_high_pc [DW_FORM_data4] ({{.*}})
 ; BB-SECTIONS: DW_AT_low_pc [DW_FORM_addr] (0x0000000000000000)
 ; BB-SECTIONS-NEXT: DW_AT_ranges [DW_FORM_sec_offset]
-; BB-SECTIONS-NEXT: [{{.*}}) ".text._Z3fooi.__part.1"
-; BB-SECTIONS-NEXT: [{{.*}}) ".text._Z3fooi.__part.2"
-; BB-SECTIONS-NEXT: [{{.*}}) ".text._Z3fooi.__part.3"
-; BB-SECTIONS-NEXT: [{{.*}}) ".text"
+; BB-SECTIONS-NEXT: [{{.*}}) ".text._Z3fooi._Z3fooi.__part.1"
+; BB-SECTIONS-NEXT: [{{.*}}) ".text._Z3fooi._Z3fooi.__part.2"
+; BB-SECTIONS-NEXT: [{{.*}}) ".text._Z3fooi._Z3fooi.__part.3"
+; BB-SECTIONS-NEXT: [{{.*}}) ".text._Z3fooi"
 ; BB-SECTIONS-ASM: _Z3fooi:
 ; BB-SECTIONS-ASM: .Ltmp{{[0-9]+}}:
 ; BB-SECTIONS-ASM-NEXT: .loc 1 2 9 prologue_end