From 5aafb6def54fc6f4de28e1bccc42a036ca8e33bf Mon Sep 17 00:00:00 2001
From: Dan Gohman <dan433584@gmail.com>
Date: Mon, 23 Nov 2015 19:12:37 +0000
Subject: [PATCH] [WebAssembly] Always print loop end labels

WebAssembly is currently using labels to end scopes, so for example a
loop scope looks like this:

BB0_0:
  loop BB0_1
  ...
BB0_1:

with BB0_0 being the label of the first block not in the loop. This
requires that the label be printed even when it's only reachable via
fallthrough. To arrange this, insert a no-op LOOP_END instruction in
such cases at the end of the loop.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253901 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp  |  4 ++++
 lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp | 12 +++++++++++-
 lib/Target/WebAssembly/WebAssemblyInstrControl.td |  5 +++++
 test/CodeGen/WebAssembly/cfg-stackify.ll          |  3 +++
 4 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp b/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
index c452e1dd024..f7f37ee8c89 100644
--- a/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
+++ b/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
@@ -221,6 +221,10 @@ void WebAssemblyAsmPrinter::EmitInstruction(const MachineInstr *MI) {
     // These represent values which are live into the function entry, so there's
     // no instruction to emit.
     break;
+  case WebAssembly::LOOP_END:
+    // This is a no-op which just exists to tell AsmPrinter.cpp that there's a
+    // fallthrough which nevertheless requires a label for the destination here.
+    break;
   default: {
     WebAssemblyMCInstLower MCInstLowering(OutContext, *this);
     MCInst TmpInst;
diff --git a/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp b/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
index 78e15f036b2..2ec1ea5f65a 100644
--- a/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
+++ b/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
@@ -289,11 +289,21 @@ static void PlaceMarkers(MachineFunction &MF, const MachineLoopInfo &MLI,
         MachineBasicBlock *Bottom = Loop->getBottomBlock();
         auto Iter = next(MachineFunction::iterator(Bottom));
         if (Iter == MF.end()) {
-          MF.push_back(MF.CreateMachineBasicBlock());
+          MachineBasicBlock *Label = MF.CreateMachineBasicBlock();
+          // Give it a fake predecessor so that AsmPrinter prints its label.
+          Label->addSuccessor(Label);
+          MF.push_back(Label);
           Iter = next(MachineFunction::iterator(Bottom));
         }
         BuildMI(MBB, MBB.begin(), DebugLoc(), TII.get(WebAssembly::LOOP))
             .addMBB(&*Iter);
+
+        // Emit a special no-op telling the asm printer that we need a label
+        // to close the loop scope, even though the destination is only
+        // reachable by fallthrough.
+        if (!Bottom->back().isBarrier())
+          BuildMI(*Bottom, Bottom->end(), DebugLoc(),
+                  TII.get(WebAssembly::LOOP_END));
       }
 
     // Place the BLOCK for MBB if MBB is branched to from above.
diff --git a/lib/Target/WebAssembly/WebAssemblyInstrControl.td b/lib/Target/WebAssembly/WebAssemblyInstrControl.td
index 92393d435c2..7fa4c5613e9 100644
--- a/lib/Target/WebAssembly/WebAssemblyInstrControl.td
+++ b/lib/Target/WebAssembly/WebAssemblyInstrControl.td
@@ -39,6 +39,11 @@ def TABLESWITCH_I64 : I<(outs), (ins I64:$index, variable_ops),
 def BLOCK     : I<(outs), (ins bb_op:$dst), [], "block   \t$dst">;
 def LOOP      : I<(outs), (ins bb_op:$dst), [], "loop    \t$dst">;
 
+// No-op to indicate to the AsmPrinter that a loop ends here, so a
+// basic block label is needed even if it wouldn't otherwise appear so.
+let isTerminator = 1, hasCtrlDep = 1 in
+def LOOP_END : I<(outs), (ins), []>;
+
 multiclass RETURN<WebAssemblyRegClass vt> {
   def RETURN_#vt : I<(outs), (ins vt:$val), [(WebAssemblyreturn vt:$val)],
                      "return  \t$val">;
diff --git a/test/CodeGen/WebAssembly/cfg-stackify.ll b/test/CodeGen/WebAssembly/cfg-stackify.ll
index cac98199215..e62a8c88ef9 100644
--- a/test/CodeGen/WebAssembly/cfg-stackify.ll
+++ b/test/CodeGen/WebAssembly/cfg-stackify.ll
@@ -186,6 +186,7 @@ entry:
 ; CHECK: BB7_1:
 ; CHECK: i32.store $0, $pop{{[0-9]+}}{{$}}
 ; CHECK: br BB7_1{{$}}
+; CHECK: BB7_2:
 define i32 @minimal_loop(i32* %p) {
 entry:
   store volatile i32 0, i32* %p
@@ -200,6 +201,7 @@ loop:
 ; CHECK: BB8_1:
 ; CHECK: loop BB8_2{{$}}
 ; CHECK: br_if $pop{{[0-9]+}}, BB8_1{{$}}
+; CHECK: BB8_2:
 ; CHECK: return ${{[0-9]+}}{{$}}
 define i32 @simple_loop(i32* %p, i32 %a) {
 entry:
@@ -285,6 +287,7 @@ exit:
 ; CHECK: BB11_5:
 ; CHECK: BB11_6:
 ; CHECK: br              BB11_1{{$}}
+; CHECK: BB11_7:
 define i32 @doublediamond_in_a_loop(i32 %a, i32 %b, i32* %p) {
 entry:
   br label %header