[WebAssembly] Fix conflict between ret legalization and sjlj

Summary:
When the WebAssembly backend encounters a return type that doesn't
fit within i32, SelectionDAG performs sret demotion, adding an
additional argument to the start of the function that contains
a pointer to an sret buffer to use instead. However, this conflicts
with the emscripten sjlj lowering pass. There we translate calls like:

```
	call {i32, i32} @foo()
```

into (in pseudo-llvm)
```
	%addr = @foo
	call {i32, i32} @__invoke_{i32,i32}(%addr)
```

i.e. we perform an indirect call through an extra function.
However, the sret transform now transforms this into
the equivalent of
```
        %addr = @foo
        %sret = alloca {i32, i32}
        call {i32, i32} @__invoke_{i32,i32}(%sret, %addr)
```
(while simultaneously translation the implementation of @foo as well).
Unfortunately, this doesn't work out. The __invoke_ ABI expected
the function address to be the first argument, causing crashes.

There is several possible ways to fix this:
1. Implementing the sret rewrite at the IR level as well and performing
   it as part of lowering to __invoke
2. Fixing the wasm backend to recognize that __invoke has a special ABI
3. A change to the binaryen/emscripten ABI to recognize this situation

This revision implements the middle option, teaching the backend to
treat __invoke_ functions specially in sret lowering. This is achieved
by
1) Introducing a new CallingConv ID for invoke functions
2) When this CallingConv ID is seen in the backend and the first argument
   is marked as sret (a function pointer would never be marked as sret),
   swapping the first two arguments.

Reviewed By: tlively, aheejin
Differential Revision: https://reviews.llvm.org/D65463

llvm-svn: 367935

llvm/include/llvm/IR/CallingConv.h

@@ -222,6 +222,14 @@ namespace CallingConv {
     // Calling convention between AArch64 Advanced SIMD functions
     AArch64_VectorCall = 97,
 
+    /// Calling convention between AArch64 SVE functions
+    AArch64_SVE_VectorCall = 98,
+
+    /// Calling convention for emscripten __invoke_* functions. The first
+    /// argument is required to be the function ptr being indirectly called.
+    /// The remainder matches the regular calling convention.
+    WASM_EmscriptenInvoke = 99,
+
     /// The highest possible calling convention ID. Must be some 2^k - 1.
     MaxID = 1023
   };

llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp

@@ -613,7 +613,8 @@ static bool callingConvSupported(CallingConv::ID CallConv) {
          CallConv == CallingConv::Cold ||
          CallConv == CallingConv::PreserveMost ||
          CallConv == CallingConv::PreserveAll ||
-         CallConv == CallingConv::CXX_FAST_TLS;
+         CallConv == CallingConv::CXX_FAST_TLS ||
+         CallConv == CallingConv::WASM_EmscriptenInvoke;
 }
 
 SDValue
@@ -649,6 +650,16 @@ WebAssemblyTargetLowering::LowerCall(CallLoweringInfo &CLI,
 
   SmallVectorImpl<ISD::OutputArg> &Outs = CLI.Outs;
   SmallVectorImpl<SDValue> &OutVals = CLI.OutVals;
+
+  // The generic code may have added an sret argument. If we're lowering an
+  // invoke function, the ABI requires that the function pointer be the first
+  // argument, so we may have to swap the arguments.
+  if (CallConv == CallingConv::WASM_EmscriptenInvoke && Outs.size() >= 2 &&
+      Outs[0].Flags.isSRet()) {
+    std::swap(Outs[0], Outs[1]);
+    std::swap(OutVals[0], OutVals[1]);
+  }
+
   unsigned NumFixedArgs = 0;
   for (unsigned I = 0; I < Outs.size(); ++I) {
     const ISD::OutputArg &Out = Outs[I];

llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp

@@ -418,7 +418,7 @@ Value *WebAssemblyLowerEmscriptenEHSjLj::wrapInvoke(CallOrInvoke *CI) {
   Args.append(CI->arg_begin(), CI->arg_end());
   CallInst *NewCall = IRB.CreateCall(getInvokeWrapper(CI), Args);
   NewCall->takeName(CI);
-  NewCall->setCallingConv(CI->getCallingConv());
+  NewCall->setCallingConv(CallingConv::WASM_EmscriptenInvoke);
   NewCall->setDebugLoc(CI->getDebugLoc());
 
   // Because we added the pointer to the callee as first argument, all

llvm/test/CodeGen/WebAssembly/lower-em-exceptions-whitelist.ll

@@ -38,7 +38,7 @@ entry:
           to label %invoke.cont unwind label %lpad
 ; CHECK: entry:
 ; CHECK-NEXT: store i32 0, i32*
-; CHECK-NEXT: call void @__invoke_void(void ()* @foo)
+; CHECK-NEXT: call cc{{.*}} void @__invoke_void(void ()* @foo)
 
 invoke.cont:                                      ; preds = %entry
   br label %try.cont

llvm/test/CodeGen/WebAssembly/lower-em-exceptions.ll

@@ -16,7 +16,7 @@ entry:
           to label %invoke.cont unwind label %lpad
 ; CHECK: entry:
 ; CHECK-NEXT: store i32 0, i32* @__THREW__
-; CHECK-NEXT: call void @__invoke_void_i32(void (i32)* @foo, i32 3)
+; CHECK-NEXT: call cc{{.*}} void @__invoke_void_i32(void (i32)* @foo, i32 3)
 ; CHECK-NEXT: %[[__THREW__VAL:.*]] = load i32, i32* @__THREW__
 ; CHECK-NEXT: store i32 0, i32* @__THREW__
 ; CHECK-NEXT: %cmp = icmp eq i32 %[[__THREW__VAL]], 1
@@ -72,7 +72,7 @@ entry:
           to label %invoke.cont unwind label %lpad
 ; CHECK: entry:
 ; CHECK-NEXT: store i32 0, i32* @__THREW__
-; CHECK-NEXT: call void @__invoke_void_i32(void (i32)* @foo, i32 3)
+; CHECK-NEXT: call cc{{.*}} void @__invoke_void_i32(void (i32)* @foo, i32 3)
 ; CHECK-NEXT: %[[__THREW__VAL:.*]] = load i32, i32* @__THREW__
 ; CHECK-NEXT: store i32 0, i32* @__THREW__
 ; CHECK-NEXT: %cmp = icmp eq i32 %[[__THREW__VAL]], 1
@@ -123,7 +123,7 @@ entry:
           to label %invoke.cont unwind label %lpad
 ; CHECK: entry:
 ; CHECK-NEXT: store i32 0, i32* @__THREW__
-; CHECK-NEXT: %0 = call noalias i8* @"__invoke_i8*_i8_i8"(i8* (i8, i8)* @bar, i8 signext 1, i8 zeroext 2)
+; CHECK-NEXT: %0 = call cc{{.*}} noalias i8* @"__invoke_i8*_i8_i8"(i8* (i8, i8)* @bar, i8 signext 1, i8 zeroext 2)
 
 invoke.cont:                                      ; preds = %entry
   br label %try.cont

llvm/test/CodeGen/WebAssembly/lower-em-sjlj-sret.ll

@@ -0,0 +1,27 @@
+; RUN: llc < %s -asm-verbose=false -enable-emscripten-sjlj -wasm-keep-registers | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown"
+
+%struct.__jmp_buf_tag = type { [6 x i32], i32, [32 x i32] }
+
+declare i32 @setjmp(%struct.__jmp_buf_tag*) #0
+declare {i32, i32} @returns_struct()
+
+; Test the combination of backend legalization of large return types and the
+; Emscripten sjlj transformation
+define {i32, i32} @legalized_to_sret() {
+entry:
+  %env = alloca [1 x %struct.__jmp_buf_tag], align 16
+  %arraydecay = getelementptr inbounds [1 x %struct.__jmp_buf_tag], [1 x %struct.__jmp_buf_tag]* %env, i32 0, i32 0
+  %call = call i32 @setjmp(%struct.__jmp_buf_tag* %arraydecay) #0
+; This is the function pointer to pass to invoke.
+; It needs to be the first argument (that's what we're testing here)
+; CHECK:	i32.const	$push[[FPTR:[0-9]+]]=, returns_struct
+; This is the sret stack region (as an offset from the stack pointer local)
+; CHECK:	call	"__invoke_{i32.i32}", $pop[[FPTR]]
+  %ret = call {i32, i32} @returns_struct()
+  ret {i32, i32} %ret
+}
+
+attributes #0 = { returns_twice }

llvm/test/CodeGen/WebAssembly/lower-em-sjlj.ll

@@ -34,7 +34,7 @@ entry:
 ; CHECK-NEXT: phi i32 [ 0, %entry ], [ %[[LONGJMP_RESULT:.*]], %if.end ]
 ; CHECK-NEXT: %[[ARRAYDECAY1:.*]] = getelementptr inbounds [1 x %struct.__jmp_buf_tag], [1 x %struct.__jmp_buf_tag]* %[[BUF]], i32 0, i32 0
 ; CHECK-NEXT: store i32 0, i32* @__THREW__
-; CHECK-NEXT: call void @"__invoke_void_%struct.__jmp_buf_tag*_i32"(void (%struct.__jmp_buf_tag*, i32)* @emscripten_longjmp_jmpbuf, %struct.__jmp_buf_tag* %[[ARRAYDECAY1]], i32 1)
+; CHECK-NEXT: call cc{{.*}} void @"__invoke_void_%struct.__jmp_buf_tag*_i32"(void (%struct.__jmp_buf_tag*, i32)* @emscripten_longjmp_jmpbuf, %struct.__jmp_buf_tag* %[[ARRAYDECAY1]], i32 1)
 ; CHECK-NEXT: %[[__THREW__VAL:.*]] = load i32, i32* @__THREW__
 ; CHECK-NEXT: store i32 0, i32* @__THREW__
 ; CHECK-NEXT: %[[CMP0:.*]] = icmp ne i32 %__THREW__.val, 0
@@ -85,7 +85,7 @@ entry:
 ; CHECK: %[[SETJMP_TABLE:.*]] = call i32* @saveSetjmp(
 
 ; CHECK: entry.split:
-; CHECK: call void @__invoke_void(void ()* @foo)
+; CHECK: @__invoke_void(void ()* @foo)
 
 ; CHECK: entry.split.split:
 ; CHECK-NEXT: %[[BUF:.*]] = bitcast i32* %[[SETJMP_TABLE]] to i8*
@@ -105,7 +105,7 @@ entry:
 
 ; CHECK: entry.split:
 ; CHECK: store i32 0, i32* @__THREW__
-; CHECK-NEXT: call void @__invoke_void(void ()* @foo)
+; CHECK-NEXT: call cc{{.*}} void @__invoke_void(void ()* @foo)
 ; CHECK-NEXT: %[[__THREW__VAL:.*]] = load i32, i32* @__THREW__
 ; CHECK-NEXT: store i32 0, i32* @__THREW__
 ; CHECK-NEXT: %[[CMP0:.*]] = icmp ne i32 %[[__THREW__VAL]], 0