[GlobalISel] Handle nullptr constants in dbg.value

[fdeazeve]

Summary

Currently, the LLVM IR -> MIR translator fails to translate dbg.values
whose first argument is a null pointer. However, in other portions of
the code, such pointers are always lowered to the constant zero, for
example see IRTranslator::Translate(Constant, Register).

This patch addresses the limitation by following the same approach of
lowering null pointers to zero.

A prior test was checking that null pointers were always lowered to
$noreg; this test is changed to check for zero, and the previous
behavior is now checked by introducing a dbg.value whose first argument
is the address of a global variable.

[fdeazeve]

Removed redundant llvm:: namespace.

[fdeazeve]

The failing test is a spurious timeout that seems to happen randomly in other reviews too: AddressSanitizer-x86_64-linux :: TestCases/scariness_score_test.cpp

llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp

MachineInstrBuilder MachineIRBuilder::buildConstDbgValue(const Constant &C,
 
   if (auto *CI = dyn_cast<ConstantInt>(NumericConstant)) {
     if (CI->getBitWidth() > 64)
       MIB.addCImm(CI);
     else
       MIB.addImm(CI->getZExtValue());
   } else if (auto *CFP = dyn_cast<ConstantFP>(NumericConstant)) {
     MIB.addFPImm(CFP);
+  } else if (isa<ConstantPointerNull>(NumericConstant)) {
+    MIB.addImm(0);
   } else {
     // Insert $noreg if we didn't find a usable constant and had to drop it.
     MIB.addReg(Register());
   }
 
   MIB.addImm(0).addMetadata(Variable).addMetadata(Expr);
   return insertInstr(MIB);
 }

llvm/test/CodeGen/AArch64/GlobalISel/debug-insts.ll

 ; CHECK: DBG_VALUE %{{[0-9]+}}(p0), 0, !14, !DIExpression(), debug-location !15
 define void @debug_declare_vla(i32 %in) #0 !dbg !13 {
 entry:
   %vla.addr = alloca i32, i32 %in
   call void @llvm.dbg.declare(metadata i32* %vla.addr, metadata !14, metadata !DIExpression()), !dbg !15
   ret void, !dbg !15
 }
 
+@gv = global i32 zeroinitializer
+
 ; CHECK-LABEL: name: debug_value
 ; CHECK: [[IN:%[0-9]+]]:_(s32) = COPY $w0
 define void @debug_value(i32 %in) #0 !dbg !16 {
   %addr = alloca i32
 ; CHECK: DBG_VALUE [[IN]](s32), $noreg, !17, !DIExpression(), debug-location !18
   call void @llvm.dbg.value(metadata i32 %in, i64 0, metadata !17, metadata !DIExpression()), !dbg !18
   store i32 %in, i32* %addr
 ; CHECK: DBG_VALUE %1(p0), $noreg, !17, !DIExpression(DW_OP_deref), debug-location !18
   call void @llvm.dbg.value(metadata i32* %addr, i64 0, metadata !17, metadata !DIExpression(DW_OP_deref)), !dbg !18
 ; CHECK: DBG_VALUE 123, 0, !17, !DIExpression(), debug-location !18
   call void @llvm.dbg.value(metadata i32 123, i64 0, metadata !17, metadata !DIExpression()), !dbg !18
 ; CHECK: DBG_VALUE float 1.000000e+00, 0, !17, !DIExpression(), debug-location !18
   call void @llvm.dbg.value(metadata float 1.000000e+00, i64 0, metadata !17, metadata !DIExpression()), !dbg !18
-; CHECK: DBG_VALUE $noreg, 0, !17, !DIExpression(), debug-location !18
+; CHECK: DBG_VALUE 0, 0, !17, !DIExpression(), debug-location !18
   call void @llvm.dbg.value(metadata i32* null, i64 0, metadata !17, metadata !DIExpression()), !dbg !18
+; CHECK: DBG_VALUE $noreg, 0, !17, !DIExpression(), debug-location !18
+  call void @llvm.dbg.value(metadata i32* @gv, i64 0, metadata !17, metadata !DIExpression()), !dbg !18
 ; CHECK: DBG_VALUE 42, 0, !17, !DIExpression(), debug-location !18
   call void @llvm.dbg.value(metadata i32* inttoptr (i64 42 to i32*), i64 0, metadata !17, metadata !DIExpression()), !dbg !18
   ret void
 }
 
 ; Function Attrs: nounwind readnone
 declare void @llvm.dbg.declare(metadata, metadata, metadata)
 declare void @llvm.dbg.value(metadata, i64, metadata, metadata)