[GlobalISel] Lower dbg.declare into indirect DBG_VALUE

[cuviper]

Summary

D31439 changed the semantics of dbg.declare to take the address of a
variable as the first argument, making it indirect. It specifically
updated FastISel for this change here:

https://reviews.llvm.org/D31439#change-WVArzi177jPl

GlobalISel needs to follow suit, or else it will be missing a level of
indirection in the generated debuginfo. This problem was seen in a Rust
debuginfo test on aarch64, since GlobalISel is used at -O0 for aarch64.

https://github.com/rust-lang/rust/issues/49807
https://bugzilla.redhat.com/show_bug.cgi?id=1611597
https://bugzilla.redhat.com/show_bug.cgi?id=1625768

[aprantl]

Note that strictly speaking a dbg.declare is not 100% the same as an indirect DBG_VALUE. A dbg.declare declares a stack slot for a variable, and specifically has a lifetime valid for the entire lexical scope of that variable, whereas the lifetime of a DBG_VALUE only lasts until the next DBG_VALUE for the same variable, the end of the basic block (except where LiveDebugValues can prove it doesn't), or until the value the DBG_Value is pointing to is clobbered, whichever happens first.

Because of this the other instruction selectors enter dbg.declares into the MMI sidetable, which is a map of stack slots.

[aprantl]

Because of LiveDebugValues this patch might still work though.

Does GlobalISel support the MMI side table? Otherwise this seems a fine approach.

[rnk]

In D51749#1226398, @aprantl wrote:

Because of LiveDebugValues this patch might still work though.

Does GlobalISel support the MMI side table? Otherwise this seems a fine approach.

It already uses the (now MachineFunction, no longer MMI) side table, if the argument is a static alloca. The original code with more context:

auto AI = dyn_cast<AllocaInst>(Address);
if (AI && AI->isStaticAlloca()) {
  // Static allocas are tracked at the MF level, no need for DBG_VALUE
  // instructions (in fact, they get ignored if they *do* exist).
  MF->setVariableDbgInfo(DI.getVariable(), DI.getExpression(),
                         getOrCreateFrameIndex(*AI), DI.getDebugLoc());
} else
  MIRBuilder.buildDirectDbgValue(getOrCreateVReg(*Address),
                                 DI.getVariable(), DI.getExpression());
return true;

So, this only affects variables that are declared in not-allocas at -O0, i.e. things passed indirectly with byval, or sret, or by address in the way that non-trivially copyable C++ types are.

This code could be improved to handle more cases where the value in question actually has a frame index. byval and inalloca are the obvious ways that I know of to get those. Looks like this is the code we use in SDAG ISel for it:

} else if (const auto *Arg = dyn_cast<Argument>(
               Address->stripInBoundsConstantOffsets())) {
  FI = FuncInfo.getArgumentFrameIndex(Arg);
}

I think this change could use a more direct test, consider starting with the IR generated for a C++ object, like:

struct NTCopy {
  NTCopy();
  NTCopy(const NTCopy &);
  int x;
};
int foo(NTCopy o) {
  return o.x;
}

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

12

This test is strange, it declares the variable twice, once in %in.addr (correct) and again in %in (incorrect).

20

This change makes sense, though. The VLA is at some stack pointer offset. I assume p0 is the SP reg.

[cuviper]

In D51749#1226417, @rnk wrote:

I think this change could use a more direct test, consider starting with the IR generated for a C++ object, like:

struct NTCopy {
  NTCopy();
  NTCopy(const NTCopy &);
  int x;
};
int foo(NTCopy o) {
  return o.x;
}

Thanks for this simple example -- it does indeed produce a bad DEBUG_VALUE of $x0 before, now fixed to an indirect [$x0+0]. I'll add this as a test.

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

12

This test is strange, it declares the variable twice, once in %in.addr (correct) and again in %in (incorrect).

Yeah, I don't know why. Clang only produces the %in.addr version for void debug_declare(int in). Then you don't get any DBG_VALUE output for this function, but I think that's probably OK, and in.addr has CHECK lines above. I can remove the plain %in one.

20

I assume p0 is the SP reg.

Yes, the lines up to %6 compute the VLA size, and then:

$sp = COPY %6(p0)
%7:_(p0) = COPY %6(p0)
DBG_VALUE debug-use %7(p0), 0, !14, !DIExpression(), debug-location !15

[cuviper]

• Removed the duplicate (and incorrect) declaration in debug-insts.ll debug_declare
• Added debug-cpp.ll which more directly targets this change.

[rnk]

lgtm

lib/CodeGen/GlobalISel/IRTranslator.cpp

       // instructions (in fact, they get ignored if they *do* exist).
       MF->setVariableDbgInfo(DI.getVariable(), DI.getExpression(),
                              getOrCreateFrameIndex(*AI), DI.getDebugLoc());
-    } else
-      MIRBuilder.buildDirectDbgValue(getOrCreateVReg(*Address),
-                                     DI.getVariable(), DI.getExpression());
+    } else {
+      // A dbg.declare describes the address of a source variable, so lower it
+      // into an indirect DBG_VALUE.
+      MIRBuilder.buildIndirectDbgValue(getOrCreateVReg(*Address),
+                                       DI.getVariable(), DI.getExpression());
+    }
     return true;
   }
   case Intrinsic::dbg_label: {

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

 ; CHECK:    - { id: {{.*}}, name: in.addr, type: default, offset: 0, size: {{.*}}, alignment: {{.*}},
 ; CHECK-NEXT: callee-saved-register: '', callee-saved-restored: true,
 ; CHECK-NEXT: debug-info-variable: '!11', debug-info-expression: '!DIExpression()',
-; CHECK: DBG_VALUE debug-use %0(s32), debug-use $noreg, !11, !DIExpression(), debug-location !12
+; CHECK: DBG_VALUE debug-use %0(s32), 0, !11, !DIExpression(), debug-location !12
 define void @debug_declare(i32 %in) #0 !dbg !7 {
 entry:
   %in.addr = alloca i32, align 4

 }
 
 ; CHECK-LABEL: name: debug_declare_vla
-; CHECK: DBG_VALUE debug-use %{{[0-9]+}}(p0), debug-use $noreg, !14, !DIExpression(), debug-location !15
+; CHECK: DBG_VALUE debug-use %{{[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