[TargetLoweringObjectFileImpl] Produce .text.hot. instead of .text.hot for -fno-unique-section-names

GNU ld's internal linker script uses (https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=add44f8d5c5c05e08b11e033127a744d61c26aee)

  .text           :
  {
    *(.text.unlikely .text.*_unlikely .text.unlikely.*)
    *(.text.exit .text.exit.*)
    *(.text.startup .text.startup.*)
    *(.text.hot .text.hot.*)
    *(SORT(.text.sorted.*))
    *(.text .stub .text.* .gnu.linkonce.t.*)
    /* .gnu.warning sections are handled specially by elf.em.  */
    *(.gnu.warning)
  }

Because `*(.text.exit .text.exit.*)` is ordered before `*(.text .text.*)`, in a -ffunction-sections build, the C library function `exit` will be placed before other functions.
gold's `-z keep-text-section-prefix` has the same problem.

In lld, `-z keep-text-section-prefix` recognizes `.text.{exit,hot,startup,unlikely,unknown}.*`, but not `.text.{exit,hot,startup,unlikely,unknown}`, to avoid the strange placement problem.

In -fno-function-sections or -fno-unique-section-names mode, a function whose `function_section_prefix` is set to `.exit"`
will go to the output section `.text` instead of `.text.exit` when linked by lld.
To address the problem, append a dot to become `.text.exit.`

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D79600

Author: MaskRay

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -611,15 +611,19 @@ getELFSectionNameForGlobal(const GlobalObject *GO, SectionKind Kind,
     Name = getSectionPrefixForGlobal(Kind);
   }
 
+  bool HasPrefix = false;
   if (const auto *F = dyn_cast<Function>(GO)) {
-    if (Optional<StringRef> Prefix = F->getSectionPrefix())
+    if (Optional<StringRef> Prefix = F->getSectionPrefix()) {
       Name += *Prefix;
+      HasPrefix = true;
+    }
   }
 
   if (UniqueSectionName) {
     Name.push_back('.');
     TM.getNameWithPrefix(Name, GO, Mang, /*MayAlwaysUsePrivate*/true);
-  }
+  } else if (HasPrefix)
+    Name.push_back('.');
   return Name;
 }
 

llvm/test/Transforms/CodeGenPrepare/X86/section.ll

@@ -1,10 +1,14 @@
 ; RUN: opt < %s -codegenprepare -S | FileCheck %s
+; RUN: llc < %s | FileCheck --check-prefix=ASM1 %s
+; RUN: llc < %s -function-sections | FileCheck --check-prefix=ASM2 %s
 
 target triple = "x86_64-pc-linux-gnu"
 
 ; This tests that hot/cold functions get correct section prefix assigned
 
 ; CHECK: hot_func1{{.*}}!section_prefix ![[HOT_ID:[0-9]+]]
+; ASM1: .section .text.hot.,"ax",@progbits
+; ASM2: .section .text.hot.hot_func1,"ax",@progbits
 ; The entry is hot
 define void @hot_func1() !prof !15 {
   ret void
@@ -40,6 +44,8 @@ for.end:
 ; not call site VP metadata (which can exist on value profiled memcpy,
 ; or possibly left behind after static analysis based devirtualization).
 ; CHECK: cold_func1{{.*}}!section_prefix ![[COLD_ID:[0-9]+]]
+; ASM1: .section .text.unlikely.,"ax",@progbits
+; ASM2: .section .text.unlikely.cold_func1,"ax",@progbits
 define void @cold_func1() !prof !16 {
   call void @hot_func1(), !prof !17
   call void @hot_func1(), !prof !17