[MIParser] Make sure that getHexUint doesn't produce APInts with a bitwidth of 0

[paquette]

Summary

If getHexUint reads in a 0 in hex, it will create an APInt A with a value of 0. It will then use that APInt to populate Result. It calculates the bitwidth of Result using A.getActiveBits(). This function returns the bitwidth of A - the number of leading zeroes in A.

A is 0, so all of its bits are leading 0s. Thus, we try to construct Result with a bitwidth of 0, hitting an assert.

This appeared originally by compiling test-suite/SingleSource/Benchmarks/Misc/mandel.c like so:

clang -Oz -mno-red-zone -S -emit-llvm test-suite/SingleSource/Benchmarks/Misc/mandel.c -o /tmp/mandel.ll
llc -march=aarch64 -stop-after=patchable-function /tmp/mandel.ll -o /tmp/mandel.mir
llc -march=aarch64 /tmp/mandel.mir

If you have asserts enabled, this blows up because of a successor basic block being tagged with a probability of 0.

[MatzeB]

LGTM.

[MatzeB]

Looks like this was committed in r312387

lib/CodeGen/MIRParser/MIParser.cpp

bool MIParser::getHexUint(APInt &Result) {
   assert(Token.is(MIToken::HexLiteral));
   StringRef S = Token.range();
   assert(S[0] == '0' && tolower(S[1]) == 'x');
   // This could be a floating point literal with a special prefix.
   if (!isxdigit(S[2]))
     return true;
   StringRef V = S.substr(2);
   APInt A(V.size()*4, V, 16);
-  Result = APInt(A.getActiveBits(),
-                 ArrayRef<uint64_t>(A.getRawData(), A.getNumWords()));
+
+  // If A is 0, then A.getActiveBits() is 0. This isn't a valid bitwidth. Make
+  // sure it isn't the case before constructing result.
+  unsigned NumBits = (A == 0) ? 32 : A.getActiveBits();
+  Result = APInt(NumBits, ArrayRef<uint64_t>(A.getRawData(), A.getNumWords()));
   return false;
 }
 
 bool MIParser::parseMemoryOperandFlag(MachineMemOperand::Flags &Flags) {
   const auto OldFlags = Flags;
   switch (Token.kind()) {
   case MIToken::kw_volatile:
     Flags |= MachineMemOperand::MOVolatile;

test/CodeGen/Generic/zero-probability.mir

+# RUN: llc -o /dev/null %s 
+# REQUIRES: asserts
+# Makes sure that having a probability of 0x00000000 to branch to a successor
+# doesn't hit an APInt assert in the MIParser.
+
+--- |
+  define i32 @main() local_unnamed_addr #0 {
+  entry:
+    ret i32 0
+  
+  other:
+    ret i32 0
+  }
+  
+  attributes #0 = { nounwind }
+  
+  !llvm.module.flags = !{!0, !1}
+  !llvm.ident = !{!2}
+  
+  !0 = !{i32 1, !"wchar_size", i32 4}
+  !1 = !{i32 7, !"PIC Level", i32 2}
+  !2 = !{!"clang version 6.0.0"}
+  !3 = !{!"branch_weights", i32 0, i32 -1}
+
+...
+---
+name:            main
+alignment:       2
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+tracksRegLiveness: true   
+body:             |
+  bb.0.entry:
+    successors: %bb.1.other(0x00000000)
+  bb.1.other:
+
+...