[PATCH] D142654: [LiveDebugValues] Allow EntryValue with OP_deref expressions

[Felipe de Azevedo Piovezan via Phabricator via llvm-commits]

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With D68945 <https://reviews.llvm.org/D68945>, more DBG_VALUEs were created without the indirect operand,
instead relying on OP_deref to accomplish the same effect.

At the time, however, we were not able to handle arbitrary expressions
in combination with OP_LLVM_entry_value, so D71416 <https://reviews.llvm.org/D71416> prevented the use of
such operation in the presence of expressions.

As per the comment in DIExpression::isValid, "we support only entry
values of a simple register location." As such, a simple deref operation
should be supported. In fact, D80345 <https://reviews.llvm.org/D80345> added support for indirect
DBG_VALUEs.

Taken the patches above into consideration, this commit relaxes the
restrictions on which expressions are allowed for entry value
candidates: the expression must be either empty or a single dereference
operator.

This patch is useful for D141381 <https://reviews.llvm.org/D141381>, which adds support for storing the
address of ABI-indirect parameters on the stack.

Depends on D142160 <https://reviews.llvm.org/D142160>


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D142654

Files:
  llvm/lib/CodeGen/LiveDebugValues/VarLocBasedImpl.cpp
  llvm/test/DebugInfo/MIR/X86/dbgcall-site-reference.mir


Index: llvm/test/DebugInfo/MIR/X86/dbgcall-site-reference.mir
===================================================================
--- llvm/test/DebugInfo/MIR/X86/dbgcall-site-reference.mir
+++ llvm/test/DebugInfo/MIR/X86/dbgcall-site-reference.mir
@@ -7,11 +7,7 @@
 # struct D : C { D(B); };
 # D::D(B b) : C(b) {}
 
-# Reproducer for PR44275. Ideally we should get an entry value location list
-# entry for the reference parameter b, but we are currently not able to do that
-# due to limitations in the DWARF emission code, which puts restrictions on the
-# DIExpression. For now verify that we don't crash when trying to add such an
-# entry value.
+# Reproducer for PR44275.
 
 # CHECK: DW_AT_location
 # CHECK-NEXT: [{{0x[0-9a-f]+}}, {{0x[0-9a-f]+}}): DW_OP_reg5 RDI
@@ -19,8 +15,8 @@
 # CHECK-NEXT: DW_AT_name    ("this")
 
 # CHECK: DW_AT_location
-# CHECK-NEXT: [0x0000000000000000, 0x0000000000000004): DW_OP_breg4 RSI+0)
-# TODO: Here we should ideally get a location list entry using an entry value.
+# CHECK-NEXT: [0x0000000000000000, 0x0000000000000004): DW_OP_breg4 RSI+0
+# CHECK-NEXT: [0x0000000000000004, 0x000000000000000b): DW_OP_GNU_entry_value(DW_OP_reg4 RSI))
 # CHECK-NEXT: DW_AT_name    ("b")
 
 --- |
Index: llvm/lib/CodeGen/LiveDebugValues/VarLocBasedImpl.cpp
===================================================================
--- llvm/lib/CodeGen/LiveDebugValues/VarLocBasedImpl.cpp
+++ llvm/lib/CodeGen/LiveDebugValues/VarLocBasedImpl.cpp
@@ -2151,10 +2151,9 @@
 
   // TODO: Add support for parameters that have a pre-existing debug expressions
   // (e.g. fragments).
-  if (MI.getDebugExpression()->getNumElements() > 0)
-    return false;
-
-  return true;
+  // A simple deref expression is equivalent to an indirect debug value.
+  const DIExpression *DIExpr = MI.getDebugExpression();
+  return DIExpr->isDeref() || DIExpr->getNumElements() == 0;
 }
 
 /// Collect all register defines (including aliases) for the given instruction.


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