Reland "[CodeGen] Fix unconditional branch duplication issue in bbsections"

Reverted in 4c8d056 because it broke
buildbot `llvm-clang-x86_64-expensive-checks-debian` due to the AArch64
test generating invalid code. The issue still exists, but it's fixed in
D156767, so the AArch64 test should be added there.

Differential Revision: https://reviews.llvm.org/D158674

Author: dhoekwater

llvm/lib/CodeGen/BasicBlockSections.cpp

@@ -258,7 +258,8 @@ void llvm::sortBasicBlocksAndUpdateBranches(
   [[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 &&

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

@@ -6,6 +6,7 @@
 ; 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 -split-machine-functions -O0 -mfs-psi-cutoff=0 -mfs-count-threshold=10000 | FileCheck %s -check-prefixes=MFS-O0,MFS-O0-X86
 
 ; RUN: llc < %s -mtriple=aarch64-unknown-linux-gnu -enable-split-machine-functions -mfs-allow-unsupported-triple | FileCheck %s -check-prefixes=MFS-DEFAULTS,MFS-DEFAULTS-AARCH64
 ; RUN: llc < %s -mtriple=aarch64-unknown-linux-gnu -enable-split-machine-functions -mfs-allow-unsupported-triple -mfs-psi-cutoff=0 -mfs-count-threshold=2000 | FileCheck %s --dump-input=always -check-prefixes=MFS-OPTS1,MFS-OPTS1-AARCH64
@@ -462,6 +463,29 @@ define void @foo15(i1 zeroext %0, i1 zeroext %1) nounwind !prof !27 {
   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-AARCH64:                 br
+; MFS-O0-AARCH64:                 br
+; MFS-O0-AARCH64-NOT:             br
+; MFS-O0:                     .section        .text.split.foo16
+; MFS-O0-NEXT:                foo16.cold
+  %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()