[ARM] Fix SJLJ exception handling when manually chosen on a platform where it isn't default

[compnerd]

@t.p.northover should absolutely look at this. The problem here is that the MCAsmInfo is sometimes not set up early enough which results in the exception model not being correct. We need to wire this up better so that the information is not pulled from MCAsmInfo but passed through properly.

[mstorsjo]

Reverted most of the logic back to the original form, but added a check of Options.ExceptionModel instead (which afaik always is available). If this is set to SjLj, we override UseSjLjEH, and if getMCAsmInfo() is available we verify that it matches just like before.

[compnerd]

lib/Target/ARM/ARMSubtarget.cpp

153–154

I think that I would prefer:

UsesSjLjEH = (isTargetDarwin() && !isTargetWatchABI()) || Options.ExceptionModel == ExceptionHandling::SjLj;

[mstorsjo]

lib/Target/ARM/ARMSubtarget.cpp

153–154

Sounds good. Or perhaps even UsesSjLjEH = (isTargetDarwin() && !isTargetWatchABI() && Options.ExceptionModel == ExceptionHandling::None) || Options.ExceptionModel == ExceptionHandling::SjLj; to allow disabling it on iOS as well.

lib/Target/ARM/ARMSubtarget.cpp

 
 bool ARMSubtarget::isXRaySupported() const {
   // We don't currently suppport Thumb, but Windows requires Thumb.
   return hasV6Ops() && hasARMOps() && !isTargetWindows();
 }
 
 void ARMSubtarget::initializeEnvironment() {
   // MCAsmInfo isn't always present (e.g. in opt) so we can't initialize this
-  // directly from it, but we can try to make sure they're consistent when both
-  // available.
+  // directly from it, but if present, we should use it since the type can be
+  // overridden by the user.
   UseSjLjEH = isTargetDarwin() && !isTargetWatchABI();
-  assert((!TM.getMCAsmInfo() ||
-          (TM.getMCAsmInfo()->getExceptionHandlingType() ==
-           ExceptionHandling::SjLj) == UseSjLjEH) &&
-         "inconsistent sjlj choice between CodeGen and MC");
+  if (TM.getMCAsmInfo())
+    UseSjLjEH = TM.getMCAsmInfo()->getExceptionHandlingType() == ExceptionHandling::SjLj;
 }
 
 void ARMSubtarget::initSubtargetFeatures(StringRef CPU, StringRef FS) {
   if (CPUString.empty()) {
     CPUString = "generic";
 
     if (isTargetDarwin()) {
       StringRef ArchName = TargetTriple.getArchName();

test/CodeGen/ARM/sjljehprepare-lower-empty-struct.ll

 ; RUN: llc -mtriple=armv7-apple-ios -O0 < %s | FileCheck %s
 ; RUN: llc -mtriple=armv7-apple-ios -O1 < %s | FileCheck %s
 ; RUN: llc -mtriple=armv7-apple-ios -O2 < %s | FileCheck %s
 ; RUN: llc -mtriple=armv7-apple-ios -O3 < %s | FileCheck %s
 ; RUN: llc -mtriple=armv7-apple-watchos -O3 < %s | FileCheck %s
 ; RUN: llc -mtriple=armv7k-apple-ios < %s | FileCheck %s --check-prefix=CHECK-WATCH
+; RUN: llc -mtriple=armv7-linux -exception-model sjlj -O3 < %s | FileCheck %s --check-prefix=CHECK-LINUX
 
 ; SjLjEHPrepare shouldn't crash when lowering empty structs.
 ;
 ; Checks that between in case of empty structs used as arguments
 ; nothing happens, i.e. there are no instructions between
 ; __Unwind_SjLj_Register and actual @bar invocation
 
 
 define i8* @foo(i8 %a, {} %c) personality i8* bitcast (i32 (...)* @baz to i8*) {
 entry:
 ; CHECK: bl __Unwind_SjLj_Register
 ; CHECK-NEXT: {{[A-Z][a-zA-Z0-9]*}}:
 ; CHECK-NEXT: bl _bar
+; CHECK: bl __Unwind_SjLj_Resume
+
+; CHECK-LINUX: bl _Unwind_SjLj_Register
+; CHECK-LINUX-NEXT: .{{[A-Z][a-zA-Z0-9]*}}:
+; CHECK-LINUX-NEXT: bl bar
+; CHECK-LINUX: bl _Unwind_SjLj_Resume
 
 ; CHECK-WATCH-NOT: bl __Unwind_SjLj_Register
 
   invoke void @bar ()
     to label %unreachable unwind label %handler
 
 unreachable:
   unreachable