llc: Require an explicit -mtriple=default to target default triple

[MatzeB]

Summary

Putting this up for discussion with a single test adapted. (Before landing I have to add explicit -mtriple to hundreds of tests):

We have a huge number of tests that just specify -march=xxx or -mcpu=yyy
hence have no OS specified and target the default OS which differs
between machines resulting in nondeterministic tests.

This changes llc to refuse tests that have no target triple specified.
The default can still be targeted with -mtriple=default.

[efriedma]

Having a default architecture is convenient for messing around... if we're going to do something here, I would rather just kill off "-march".

[echristo]

In D35287#807305, @efriedma wrote:

Having a default architecture is convenient for messing around... if we're going to do something here, I would rather just kill off "-march".

I'm with Eli here. I like the canonical triple idea.

-eric

[MatzeB]

In D35287#807305, @efriedma wrote:

Having a default architecture is convenient for messing around... if we're going to do something here, I would rather just kill off "-march".

• But we still have the default... you just have to use -mtriple=default to get it.
• I'd be fine with killing off -march as well.

• I guess I'll start fixing all those tests that just say -march to use -mtriple, as we need that change no matter what variant we choose here.

[MatzeB]

Ok I changed the PowerPC, X86, ARM (AArch64 wasn't necessary) targets to not use -march anymore in r309754, r309774 and r309755.
Assuming that most tests are created by copy&paste this should hopefully avoid people creating indeterministic tests that break when the default operating system changes.

So we should be fine without this change.

[evandro]

Just a couple of ¢, if I may, but I was bitten by a difference in code generation between the ELF and MachO ABIs. This is a good move.

test/CodeGen/PowerPC/vector.ll

 ; Test that vectors are scalarized/lowered correctly.
-; RUN: llc -verify-machineinstrs < %s -march=ppc32 -mcpu=g5 > %t
-; RUN: llc -verify-machineinstrs < %s -march=ppc32 -mcpu=g3 >> %t
+; RUN: llc -verify-machineinstrs < %s -mtriple=ppc32-- -mcpu=g5 > %t
+; RUN: llc -verify-machineinstrs < %s -mtriple=ppc32-- -mcpu=g3 >> %t
 
 %d8 = type <8 x double>
 %f1 = type <1 x float>
 %f2 = type <2 x float>
 %f4 = type <4 x float>
 %f8 = type <8 x float>
 %i4 = type <4 x i32>
 

tools/llc/llc.cpp

static int compileModule(char **argv, LLVMContext &Context) {
   SMDiagnostic Err;
   std::unique_ptr<Module> M;
   std::unique_ptr<MIRParser> MIR;
   Triple TheTriple;
 
   bool SkipModule = MCPU == "help" ||
                     (!MAttrs.empty() && MAttrs.front() == "help");
 
+  if (TargetTriple == "default")
+    TargetTriple = sys::getDefaultTargetTriple();
+
   // If user just wants to list available options, skip module loading
   if (!SkipModule) {
     if (InputLanguage == "mir" ||
         (InputLanguage == "" && StringRef(InputFilename).endswith(".mir"))) {
       MIR = createMIRParserFromFile(InputFilename, Err, Context);
       if (MIR)
         M = MIR->parseIRModule();
     } else
if (!SkipModule) {
     // If we are supposed to override the target triple, do so now.
     if (!TargetTriple.empty())
       M->setTargetTriple(Triple::normalize(TargetTriple));
     TheTriple = Triple(M->getTargetTriple());
   } else {
     TheTriple = Triple(Triple::normalize(TargetTriple));
   }
 
-  if (TheTriple.getTriple().empty())
-    TheTriple.setTriple(sys::getDefaultTargetTriple());
+  if (TheTriple.getTriple().empty()) {
+    errs() << argv[0] << ": No target triple specified.\n";
+    errs() << "Note: You may specify -mtriple=default for the default.\n";
+    return 1;
+  }
 
   // Get the target specific parser.
   std::string Error;
   const Target *TheTarget = TargetRegistry::lookupTarget(MArch, TheTriple,
                                                          Error);
   if (!TheTarget) {
     errs() << argv[0] << ": " << Error;
     return 1;