[PATCH] D78742: Use .text.unlikely prefix for cold MachineBasicBlock sections.

[Snehasish Kumar via Phabricator via llvm-commits]

snehasish created this revision.
snehasish added reviewers: tmsriram, mtrofin.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.

Instead of adding a ".unlikely" suffix for cold machine basic blocks,
this change updates the behaviour to use a ".text.unlikely" prefix
instead. This allows lld to group cold basic block sections together
when -z,keep-text-section-prefix is specified.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D78742

Files:
  llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
  llvm/test/CodeGen/X86/basicblock-sections-clusters-branches.ll
  llvm/test/CodeGen/X86/basicblock-sections-clusters.ll
  llvm/test/CodeGen/X86/basicblock-sections-cold.ll


Index: llvm/test/CodeGen/X86/basicblock-sections-cold.ll
===================================================================
--- llvm/test/CodeGen/X86/basicblock-sections-cold.ll
+++ llvm/test/CodeGen/X86/basicblock-sections-cold.ll
@@ -34,7 +34,7 @@
 ; Check that the basic block with id 1 doesn't get a section.
 ; LINUX-SECTIONS-NOT: .section        .text._Z3bazb.r.BB._Z3bazb,"ax", at progbits,unique
 ; Check that a single cold section is started here and id 1 and 2 blocks are placed here.
-; LINUX-SECTIONS: .section	.text._Z3bazb.unlikely,"ax", at progbits
+; LINUX-SECTIONS: .section	.text.unlikely._Z3bazb,"ax", at progbits
 ; LINUX-SECTIONS: r.BB._Z3bazb:
 ; LINUX-SECTIONS-NOT: .section        .text._Z3bazb.rr.BB._Z3bazb,"ax", at progbits,unique
 ; LINUX-SECTIONS: .LBB0_2:
Index: llvm/test/CodeGen/X86/basicblock-sections-clusters.ll
===================================================================
--- llvm/test/CodeGen/X86/basicblock-sections-clusters.ll
+++ llvm/test/CodeGen/X86/basicblock-sections-clusters.ll
@@ -48,7 +48,7 @@
 ; LINUX-SECTIONS1-LABEL:	.Ltmp0:
 ; LINUX-SECTIONS1-NEXT:		.size a.BB.foo, .Ltmp0-a.BB.foo
 ; LINUX-SECTIONS1-NOT:  	.section
-; LINUX-SECTIONS1:		.section        .text.foo.unlikely,"ax", at progbits
+; LINUX-SECTIONS1:		.section        .text.unlikely.foo,"ax", at progbits
 ; LINUX-SECTIONS1-LABEL:	raa.BB.foo:
 ; LINUX-SECTIONS1:	   	.section	.text.foo,"ax", at progbits
 ; LINUX-SECTIONS1-LABEL:	.Lfunc_end0:
Index: llvm/test/CodeGen/X86/basicblock-sections-clusters-branches.ll
===================================================================
--- llvm/test/CodeGen/X86/basicblock-sections-clusters-branches.ll
+++ llvm/test/CodeGen/X86/basicblock-sections-clusters-branches.ll
@@ -50,7 +50,7 @@
 ; LINUX-SECTIONS1:		.section        .text.foo,"ax", at progbits,unique,1
 ; LINUX-SECTIONS1-LABEL:	a.BB.foo:
 ; LINUX-SECTIONS1:		jmp raa.BB.foo
-; LINUX-SECTIONS1:		.section        .text.foo.unlikely,"ax", at progbits
+; LINUX-SECTIONS1:		.section        .text.unlikely.foo,"ax", at progbits
 ; LINUX-SECTIONS1-LABEL:	raa.BB.foo:
 
 ; LINUX-SECTIONS2:		.section        .text.foo,"ax", at progbits
Index: llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
===================================================================
--- llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -859,28 +859,31 @@
     const Function &F, const MachineBasicBlock &MBB,
     const TargetMachine &TM) const {
   assert(MBB.isBeginSection() && "Basic block does not start a section!");
-  SmallString<128> Name;
-  Name =
-      (static_cast<MCSectionELF *>(MBB.getParent()->getSection()))->getName();
   unsigned UniqueID = MCContext::GenericSectionID;
 
-  switch (MBB.getSectionID().Type) {
-    // Append suffixes to represent special cold and exception sections.
-  case MBBSectionID::SectionType::Exception:
-    Name += ".eh";
-    break;
-  case MBBSectionID::SectionType::Cold:
-    Name += ".unlikely";
-    break;
-  // For regular sections, either use a unique name, or a unique ID for the
-  // section.
-  default:
-    if (TM.getUniqueBBSectionNames()) {
-      Name += ".";
-      Name += MBB.getSymbol()->getName();
-    } else
-      UniqueID = NextUniqueID++;
-    break;
+  // For cold sections use the .text.unlikely prefix along with the parent
+  // function name. All cold blocks for the same function go to the same
+  // section. For exception blocks add a suffix to identify. For regular
+  // sections, either use a unique name, or a unique ID for the section.
+  SmallString<128> Name;
+  if (MBB.getSectionID() == MBBSectionID::ColdSectionID) {
+    Name += ".text.unlikely.";
+    Name += MBB.getParent()->getName();
+  } else {
+    Name += MBB.getParent()->getSection()->getName();
+    switch (MBB.getSectionID().Type) {
+    case MBBSectionID::SectionType::Exception: {
+      Name += ".eh";
+      break;
+    }
+    default:
+      if (TM.getUniqueBBSectionNames()) {
+        Name += ".";
+        Name += MBB.getSymbol()->getName();
+      } else
+        UniqueID = NextUniqueID++;
+      break;
+    }
   }
 
   unsigned Flags = ELF::SHF_ALLOC | ELF::SHF_EXECINSTR;


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