[MIR-Canon] Fixing case where MachineFunction is empty.

[plotfi]

Summary

This small change is to handle changes where a machine function is:

body: |
...

Currently, dereferencing the MF.begin() prior to RPO causes core dump.

[arsenm]

llvm/test/CodeGen/MIR/AArch64/print-parse-overloaded-intrinsics.mir

5–6 ↗

(On Diff #202012)

I don't see how the test is related?

[plotfi]

llvm/test/CodeGen/MIR/AArch64/print-parse-overloaded-intrinsics.mir

5–6 ↗

(On Diff #202012)

You have a good point. I will put together a separate test. This one just needs to be (empty MIR, but the IR part has a function):

--- |
  define i32 @foo() {
    ret i32 0
  }
...

[bogner]

llvm/lib/CodeGen/MIRCanonicalizerPass.cpp

107 ↗

(On Diff #202012)

MF.empty()?

108 ↗

(On Diff #202012)

Should be sufficient to write return {};

llvm/test/CodeGen/MIR/AArch64/print-parse-overloaded-intrinsics.mir

5–6 ↗

(On Diff #202012)

Agreed, it's probably better to create a simple test for this explicitly, and you should probably FileCheck that it passes the empty block through unchanged

[plotfi]

Changes based on Matt+Justin's feedback.

[bogner]

llvm/test/CodeGen/MIR/AArch64/empty-MF.mir

1–2 ↗

(On Diff #202080)

Probably best to FileCheck that we get an empty function rather than just "not crashing"

2 ↗

(On Diff #202080)

Why -verify-machineinstrs? There aren't any...

4–8 ↗

(On Diff #202080)

This is kind of a confusing way to do this - the function certainly doesn't look empty and you're relying on the fact that not providing the mir gives us something empty-ish. I'd probably drop the IR completely and write out the mir you want:

---
name: foo
body:             |
...

[plotfi]

llvm/test/CodeGen/MIR/AArch64/empty-MF.mir

4–8 ↗

(On Diff #202080)

I will give this a try. This is what was crashing the other test though.

[plotfi]

Addressing Justin's latest feedback.

llvm/trunk/lib/CodeGen/MIRCanonicalizerPass.cpp

 
 INITIALIZE_PASS_BEGIN(MIRCanonicalizer, "mir-canonicalizer",
                       "Rename Register Operands Canonically", false, false)
 
 INITIALIZE_PASS_END(MIRCanonicalizer, "mir-canonicalizer",
                     "Rename Register Operands Canonically", false, false)
 
 static std::vector<MachineBasicBlock *> GetRPOList(MachineFunction &MF) {
+  if (MF.empty())
+    return {};
   ReversePostOrderTraversal<MachineBasicBlock *> RPOT(&*MF.begin());
   std::vector<MachineBasicBlock *> RPOList;
   for (auto MBB : RPOT) {
     RPOList.push_back(MBB);
   }
 
   return RPOList;
 }

llvm/trunk/test/CodeGen/MIR/AArch64/empty-MF.mir

+# RUN: llc -mtriple aarch64-- -run-pass mir-canonicalizer %s -o - | FileCheck %s
+# CHECK: ...
+# CHECK-NEXT: ---
+# CHECK-NEXT: name:            foo
+# CHECK: body:             |
+# CHECK-EMPTY:
+# CHECK-NEXT: ...
+---
+name: foo
+body:             |
+...