[CodeGen] Fix unconditional branch duplication issue in bbsections

[dhoekwater]

Summary

If an end section basic block ends in an unconditional branch to its
fallthrough, BasicBlockSections will duplicate the unconditional branch.
This doesn't break x86, but it is a (slight) size optimization and more
importantly prevents AArch64 builds from breaking.

Ex:

bb1 (bbsections Hot):
  jmp bb2

bb2 (bbsections Cold):
  /* do work... */

After running sortBasicBlocksAndUpdateBranches():

bb1 (bbsections Hot):
  jmp bb2
  jmp bb2

bb2 (bbsections Cold):
  /* do work... */

[rahmanl]

Thanks.

llvm/test/CodeGen/Generic/machine-function-splitter.ll

9

Just -mtriple=x86_64 is enough.

llvm/lib/CodeGen/BasicBlockSections.cpp

for (auto &MBB : MF)
         MBB.setSectionID(*EHPadsSectionID);
 }
 
 void llvm::sortBasicBlocksAndUpdateBranches(
     MachineFunction &MF, MachineBasicBlockComparator MBBCmp) {
   [[maybe_unused]] const MachineBasicBlock *EntryBlock = &MF.front();
   SmallVector<MachineBasicBlock *> PreLayoutFallThroughs(MF.getNumBlockIDs());
   for (auto &MBB : MF)
-    PreLayoutFallThroughs[MBB.getNumber()] = MBB.getFallThrough();
+    PreLayoutFallThroughs[MBB.getNumber()] =
+        MBB.getFallThrough(/*JumpToFallThrough=*/false);
 
   MF.sort(MBBCmp);
   assert(&MF.front() == EntryBlock &&
          "Entry block should not be displaced by basic block sections");
 
   // Set IsBeginSection and IsEndSection according to the assigned section IDs.
   MF.assignBeginEndSections();
 

llvm/test/CodeGen/Generic/machine-function-splitter.ll

 ; REQUIRES: aarch64-registered-target
 ; REQUIRES: x86-registered-target
 
 ; COM: Machine function splitting with FDO profiles
 ; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -split-machine-functions | FileCheck %s -check-prefixes=MFS-DEFAULTS,MFS-DEFAULTS-X86
 ; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -split-machine-functions -mfs-psi-cutoff=0 -mfs-count-threshold=2000 | FileCheck %s --dump-input=always -check-prefixes=MFS-OPTS1,MFS-OPTS1-X86
 ; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -split-machine-functions -mfs-psi-cutoff=950000 | FileCheck %s -check-prefixes=MFS-OPTS2,MFS-OPTS2-X86
 ; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -split-machine-functions -mfs-split-ehcode | FileCheck %s -check-prefixes=MFS-EH-SPLIT,MFS-EH-SPLIT-X86
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -split-machine-functions -O0 -mfs-psi-cutoff=0 -mfs-count-threshold=4000 | FileCheck %s -check-prefixes=MFS-O0,MFS-O0-X86
 
 ; COM: Machine function splitting with AFDO profiles
 ; RUN: sed 's/InstrProf/SampleProfile/g' %s > %t.ll
 ; RUN: llc < %t.ll -mtriple=x86_64-unknown-linux-gnu -split-machine-functions | FileCheck %s --check-prefix=FSAFDO-MFS
 ; RUN: llc < %t.ll -mtriple=x86_64-unknown-linux-gnu -split-machine-functions | FileCheck %s --check-prefix=FSAFDO-MFS2
 ; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -debug-pass=Structure -fs-profile-file=%S/Inputs/fsloader-mfs.afdo -enable-fs-discriminator=true -improved-fs-discriminator=true -split-machine-functions 2>&1 | FileCheck %s --check-prefix=MFS_ON
 ; RUN: llc < %s -mtriple=aarch64-unknown-linux-gnu -debug-pass=Structure -fs-profile-file=%S/Inputs/fsloader-mfs.afdo -enable-fs-discriminator=true -improved-fs-discriminator=true -split-machine-functions 2>&1 | FileCheck %s --check-prefix=MFS_OFF
 
10:
   %11 = call i32 @baz()
   br label %12
 
 12:
   %13 = tail call i32 @qux()
   ret void
 }
 
+define void @foo16(i1 zeroext %0) nounwind !prof !14 !section_prefix !15 {
+;; Check that an unconditional branch is only appended to a block
+;; if it would fall through to the wrong block otherwise.
+; MFS-O0-LABEL:               foo16
+; MFS-O0-X86:                 jmp
+; MFS-O0-X86-NOT:             jmp
+; MFS-O0-X86:                 .{{LBB[a-zA-Z0-9_]*:}}
+  %2 = call i32 @baz()
+  br i1 false, label %3, label %5, !prof !25
+
+3:                                                ; preds = %1
+  %4 = call i32 @bar()
+  unreachable
+
+5:                                                ; preds = %1
+  %6 = tail call i32 @qux()
+  ret void
+}
+
 declare i32 @bar()
 declare i32 @baz()
 declare i32 @bam()
 declare i32 @qux()
 declare void @_Z1fv()
 declare i32 @__gxx_personality_v0(...)
 
 @_ZTIi = external constant ptr