[WebAssembly] Generate unreachable after __stack_chk_fail

[aheejin]

Summary

__stack_chk_fail does not return, but unreachable was not generated
following call __stack_chk_fail. This had a possibility to generate an
invalid binary for functions with a return type, because
__stack_chk_fail's return type is void and call __stack_chk_fail can
be the last instruction in the function whose return type is non-void.
Generating unreachable after it makes sure CFGStackify's
fixEndsAtEndOfFunction handles it correctly.

[aheejin]

• Align FileCheck lines

llvm/test/CodeGen/WebAssembly/stack-protector.ll

4

After adding unreachable, somehow the branch order changed and this became i32.eq instead, so I deleted this.

[tlively]

LGTM!

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

SelectionDAGBuilder::visitSPDescriptorFailure(StackProtectorDescriptor &SPD) {
   SDValue Chain =
       TLI.makeLibCall(DAG, RTLIB::STACKPROTECTOR_CHECK_FAIL, MVT::isVoid,
                       None, CallOptions, getCurSDLoc()).second;
   // On PS4, the "return address" must still be within the calling function,
   // even if it's at the very end, so emit an explicit TRAP here.
   // Passing 'true' for doesNotReturn above won't generate the trap for us.
   if (TM.getTargetTriple().isPS4CPU())
     Chain = DAG.getNode(ISD::TRAP, getCurSDLoc(), MVT::Other, Chain);
+  // WebAssembly needs an unreachable instruction after a non-returning call,
+  // because the function return type can be different from __stack_chk_fail's
+  // return type (void).
+  if (TM.getTargetTriple().isWasm())
+    Chain = DAG.getNode(ISD::TRAP, getCurSDLoc(), MVT::Other, Chain);
 
   DAG.setRoot(Chain);
 }
 
 /// visitBitTestHeader - This function emits necessary code to produce value
 /// suitable for "bit tests"
 void SelectionDAGBuilder::visitBitTestHeader(BitTestBlock &B,
                                              MachineBasicBlock *SwitchBB) {

llvm/test/CodeGen/WebAssembly/stack-protector.ll

 ; RUN: llc -verify-machineinstrs -mtriple=wasm32-unknown-unknown < %s | FileCheck -check-prefix=WASM32 %s
 
-; WASM32: i32.load        28
-; WASM32-NEXT: i32.ne
-; WASM32-NEXT: br_if           0
-
-; WASM32: __stack_chk_fail
-
 @"\01LC" = internal constant [11 x i8] c"buf == %s\0A\00"    ; <[11 x i8]*> [#uses=1]
 
+; WASM32-LABEL: test
+; WASM32: i32.load        28
+; WASM32: br_if           0
+; WASM32: call __stack_chk_fail
+; WASM32-NEXT: unreachable
+
 define void @test(i8* %a) nounwind ssp {
 entry:
   %a_addr = alloca i8*    ; <i8**> [#uses=2]
   %buf = alloca [8 x i8]    ; <[8 x i8]*> [#uses=2]
   %"alloca point" = bitcast i32 0 to i32    ; <i32> [#uses=0]
   store i8* %a, i8** %a_addr
   %buf1 = bitcast [8 x i8]* %buf to i8*    ; <i8*> [#uses=1]
   %0 = load i8*, i8** %a_addr, align 4    ; <i8*> [#uses=1]
   %1 = call i8* @strcpy(i8* %buf1, i8* %0) nounwind    ; <i8*> [#uses=0]
   %buf2 = bitcast [8 x i8]* %buf to i8*    ; <i8*> [#uses=1]
   %2 = call i32 (i8*, ...) @printf(i8* getelementptr ([11 x i8], [11 x i8]* @"\01LC", i32 0, i32 0), i8* %buf2) nounwind    ; <i32> [#uses=0]
   br label %return
 
 return:   ; preds = %entry
   ret void
 }
 
+; WASM32-LABEL: test_return_i32
+; WASM32:      call __stack_chk_fail
+; WASM32-NEXT: unreachable
+
+define i32 @test_return_i32(i8* %a) nounwind ssp {
+entry:
+  %a_addr = alloca i8*    ; <i8**> [#uses=2]
+  %buf = alloca [8 x i8]    ; <[8 x i8]*> [#uses=2]
+  %"alloca point" = bitcast i32 0 to i32    ; <i32> [#uses=0]
+  store i8* %a, i8** %a_addr
+  %buf1 = bitcast [8 x i8]* %buf to i8*    ; <i8*> [#uses=1]
+  %0 = load i8*, i8** %a_addr, align 4    ; <i8*> [#uses=1]
+  %1 = call i8* @strcpy(i8* %buf1, i8* %0) nounwind    ; <i8*> [#uses=0]
+  %buf2 = bitcast [8 x i8]* %buf to i8*    ; <i8*> [#uses=1]
+  %2 = call i32 (i8*, ...) @printf(i8* getelementptr ([11 x i8], [11 x i8]* @"\01LC", i32 0, i32 0), i8* %buf2) nounwind    ; <i32> [#uses=0]
+  br label %return
+
+return:    ; preds = %entry
+  ret i32 0
+}
+
 declare i8* @strcpy(i8*, i8*) nounwind
 
 declare i32 @printf(i8*, ...) nounwind