This patch fixes PR40587.
When a dbg.value instrinsic is emitted to the DAG
by using EmitFuncArgumentDbgValue the resulting
DBG_VALUE is hoisted to the beginning of the entry
block. I think the idea is to be able to locate
a formal argument already from the start of the
However, EmitFuncArgumentDbgValue only checked that
the value that was used to describe a variable was
originating from a function parameter, not that the
variable itself actually was an argument to the
function. So when for example assigning a local
variable "local" the value from an argument "a",
the DBG_VALUE instruction describing that "local"
had the value of "a" would be hoisted to the
beginning of the function, even if the scope for
"local" started somewhere else (or if "local"
had other values earlier in the function).
This patch adds some logic to EmitFuncArgumentDbgValue
to check that the variable being described actually
is an argument to the function. And that the dbg.value
being lowered already is in the entry block. Otherwise
we bail out, and the dbg.value will be handled as an
ordinary dbg.value (not as a "FuncArgumentDbgValue").
A tricky situation is when both the variable and
the value is related to function arguments, but not
the same argument. This is kind of hard to check.
Instead we verify that we do not describe the same
argument more than once as a "FuncArgumentDbgValue".
This solution works as long as opt has injected a
"first" dbg.value that corresponds to the formal