[XCOFF][AIX] Put each jump table in an independent section if -ffunction-sections is specified

[jasonliu]

Summary

If a function is in a unique section, putting all jump tables in .rodata will prevent functions that have a jump table to get garbage collect by the linker.
Therefore, we need to put jump table into a unique section as well.

[jasonliu]

Gentle ping.

[Xiangling_L]

LGTM. thanks

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

MCSection *TargetLoweringObjectFileXCOFF::SelectSectionForGlobal(
   if (Kind.isReadOnly())
     return ReadOnlySection;
 
   report_fatal_error("XCOFF other section types not yet implemented.");
 }
 
 MCSection *TargetLoweringObjectFileXCOFF::getSectionForJumpTable(
     const Function &F, const TargetMachine &TM) const {
-  assert (!TM.getFunctionSections() && "Unique sections not supported on XCOFF"
-          " yet.");
   assert (!F.getComdat() && "Comdat not supported on XCOFF.");
-  //TODO: Enable emiting jump table to unique sections when we support it.
+
+  if (!TM.getFunctionSections())
     return ReadOnlySection;
+
+  // If the function can be removed, produce a unique section so that
+  // the table doesn't prevent the removal.
+  SmallString<128> NameStr(".rodata.jmp..");
+  getNameWithPrefix(NameStr, &F, TM);
+  return getContext().getXCOFFSection(NameStr, XCOFF::XMC_RO, XCOFF::XTY_SD,
+                                      XCOFF::C_HIDEXT,
+                                      SectionKind::getReadOnly());
 }
 
 bool TargetLoweringObjectFileXCOFF::shouldPutJumpTableInFunctionSection(
     bool UsesLabelDifference, const Function &F) const {
   return false;
 }
 
 /// Given a mergeable constant with the specified size and relocation

llvm/test/CodeGen/PowerPC/aix-lower-jump-table.ll

 ; RUN: --check-prefixes=32LARGE-ASM,CHECK %s
 
 ; RUN: llc -mtriple powerpc64-ibm-aix-xcoff -code-model=small < %s | FileCheck \
 ; RUN: --check-prefixes=64SMALL-ASM,CHECK %s
 
 ; RUN: llc -mtriple powerpc64-ibm-aix-xcoff -code-model=large < %s | FileCheck \
 ; RUN: --check-prefixes=64LARGE-ASM,CHECK %s
 
+; RUN: llc -mtriple powerpc-ibm-aix-xcoff -function-sections < %s | FileCheck \
+; RUN: --check-prefixes=FUNC-ASM,CHECK %s
+
+; RUN: llc -mtriple powerpc64-ibm-aix-xcoff -function-sections < %s | FileCheck \
+; RUN: --check-prefixes=FUNC-ASM,CHECK %s
+
   define i32 @jump_table(i32 %a) {
   entry:
     switch i32 %a, label %sw.epilog [
       i32 1, label %sw.bb
       i32 2, label %sw.bb1
       i32 3, label %sw.bb2
       i32 4, label %sw.bb3
     ]

 ; 64LARGE-ASM:      .csect .rodata[RO],2
 ; 64LARGE-ASM:      .align  2
 ; 64LARGE-ASM: L..JTI0_0:
 ; 64LARGE-ASM:      .vbyte  4, L..BB0_2-L..JTI0_0
 ; 64LARGE-ASM:      .vbyte  4, L..BB0_3-L..JTI0_0
 ; 64LARGE-ASM:      .vbyte  4, L..BB0_4-L..JTI0_0
 ; 64LARGE-ASM:      .vbyte  4, L..BB0_5-L..JTI0_0
 
+; FUNC-ASM:         .csect .jump_table[PR],2
+; FUNC-ASM: L..BB0_2:
+; FUNC-ASM: L..BB0_3:
+; FUNC-ASM: L..BB0_4:
+; FUNC-ASM: L..BB0_5:
+; FUNC-ASM: L..BB0_6:
+; FUNC-ASM:         li 3, 0
+; FUNC-ASM:         blr
+; FUNC-ASM:         .csect .rodata.jmp..jump_table[RO],2
+; FUNC-ASM:         .align  2
+; FUNC-ASM: L..JTI0_0:
+; FUNC-ASM:         .vbyte  4, L..BB0_2-L..JTI0_0
+; FUNC-ASM:         .vbyte  4, L..BB0_3-L..JTI0_0
+; FUNC-ASM:         .vbyte  4, L..BB0_4-L..JTI0_0
+; FUNC-ASM:         .vbyte  4, L..BB0_5-L..JTI0_0
+
 ; CHECK: .toc
 ; CHECK: .tc L..JTI0_0[TC],L..JTI0_0