Re: [Dwarf-Discuss] Use of Location Description operations in DWARF Expressions?
> DW_OP_implicit_value and DW_OP_stack_value produce values (that is > R-values), not locations. I might be able to read > DW_OP_implicit_pointer as providing a location; I'm not sure. No, they don't produce a value. The expression that precedes them produces a value, and these operators produce a location description for that value. > As I said, values and locations are muddled. If you think that a > variable which has been eliminated has a location which is described by > a DW_OP_implicit_value, then we aren't working with the same definition > of "location". Evidently not. For values that don't have a location, we have these operators that provide a location description anyway. Even though they're not real locations in the sense of existing in memory or a register, they are locations in the sense that they are described by a location description. Perhaps it would help to think of "locations" as what you think of as locations, and "location descriptions" as something a bit more general. A location description can describe a real location, or it can describe something more ethereal. > This doesn't work with any of the Implicit Location Descriptions, > because there isn't any "there" there. They don't result in locations; > they give the values which would be found if the variable did exist. So > they implicitly perform the dereference operation. Quite the opposite; they perform the inverse of a dereference operation. They provide something at the level of a location description that can be dereferenced but has no other purpose. -cary ___ Dwarf-Discuss mailing list Dwarf-Discuss@lists.dwarfstd.org http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
Re: [Dwarf-Discuss] Use of Location Description operations in DWARF Expressions?
On 3/23/20 3:58 PM, Cary Coutant via Dwarf-Discuss wrote: I think that the description has become a bit less clear with the addition of the Implicit Location Descriptions in Section 2.6.1.1.4, which do compute values, rather than locations. Perhaps these should have been described in Section 2.5 as parts of a DWARF expression, not as parts of a Location Description. Strongly disagree. The whole point of an implicit location description is to give a (pseudo/virtual) location to a value that has no location. A DWARF *expression* has no use for these operators. DW_OP_implicit_value and DW_OP_stack_value produce values (that is R-values), not locations. I might be able to read DW_OP_implicit_pointer as providing a location; I'm not sure. As I said, values and locations are muddled. If you think that a variable which has been eliminated has a location which is described by a DW_OP_implicit_value, then we aren't working with the same definition of "location". The problem is that the basic DWARF concept is that a variable has a location and that you can fetch its values from this location. Thus, we expect to be able to evaluate a location description and dereference it to get a value. Similarly, you can change a variable's value by modifying its location. This dereference or modification may be a bit complicated if DW_OP_piece is involved, but the operation is symmetric. This I might call a "real" location description. This doesn't work with any of the Implicit Location Descriptions, because there isn't any "there" there. They don't result in locations; they give the values which would be found if the variable did exist. So they implicitly perform the dereference operation. But, in contrast to "real" location descriptions, the operation is not symmetric, you can't change the variable's value. ___ Dwarf-Discuss mailing list Dwarf-Discuss@lists.dwarfstd.org http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
Re: [Dwarf-Discuss] Use of Location Description operations in DWARF Expressions?
> I think that the description has become a bit less clear with the > addition of the Implicit Location Descriptions in Section 2.6.1.1.4, > which do compute values, rather than locations. Perhaps these should > have been described in Section 2.5 as parts of a DWARF expression, not > as parts of a Location Description. Strongly disagree. The whole point of an implicit location description is to give a (pseudo/virtual) location to a value that has no location. A DWARF *expression* has no use for these operators. > The description (and implementation) of DWARF expressions and locations > are somewhat muddled together. This can be seen in the first sentence > of Section 2.5: > DWARF expressions describe how to compute a value or specify a > location. > A clearer definition would specify that the DWARF expression only > computes a value, and leaving what that value means (e.g., > register/memory contents, arbitrary computation, memory address) to the > context in which the expression is used. A more precise definition of a > location, especially a composite location, would help. > > Part of this is historical, with the expression evaluation initially > used to compute a memory address and then extended to accommodate > composite locations and further extended to compute arbitrary values. > These extensions were in divergent directions, affecting both location > composition and value computation. It would have been better if the two > concepts were clearly distinguished. Other than that first sentence of 2.5, I think they are pretty clearly distinguished. Location descriptions, covered by Section 2.6, are built on top of DWARF expressions, and provide additional operators for specifying locations other than simple memory locations. -cary ___ Dwarf-Discuss mailing list Dwarf-Discuss@lists.dwarfstd.org http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
Re: [Dwarf-Discuss] Use of Location Description operations in DWARF Expressions?
I recall this being intentional as well. This is how I think of these items. And this is just the gist of things. I didn't put on my ABI Lawyer hat for this: A DWARF expression is a stack machine that evaluates to a value. A location description describes the "location" of an object. A "location" is pure concept here, and doesn't necessarily require any physical location. It can be: * In memory at an address: It has a DWARF expression which computes the start address. * In a register: DW_OP_regN, DW_OP_regx. No locdesc needed. * Nowhere, but with a known value: DW_OP_implicit_value, DW_OP_stack_value. I think of this as being an ephemeral "location", which in concrete terms would be in a buffer in the debugger or some other consumer. There's a DWARF expression which computes the value. * Nowhere, but where it's an optimized-away pointer, and its designated (pointed-to) value is a known value, much like above. * Spread out across multiple distinct locations: DW_OP_piece's, where each piece can be any one of the above. Oh, and one more: * Nowhere at all. Go fish. So locdescs can use a DWARF expression for a couple different purposes, or even multiple DWARF expressions. But they have additional operators for additional cases (e.g. registers), and for "glue" (DW_OP_piece). Conversely, DWARF expressions cannot use any of the locdesc special operators or "glue". But, of course, there could be use cases where some of these would make sense in a DWARF expression, and we just didn't think of it. Nothing springs to my mind right now... But if you have a compelling case, we certainly could move some of those special/glue operators from the locdesc category to the DWARF expression category. It think it feels a little blurry only because locdescs came first, and then we co-opted them for DWARF expressions, and restricted the use of certain operators in that case. And then it eventually changed into what we have now. But a lot of us remember the history, which creates that blur. -- Todd Allen Concurrent Real-Time On Mon, Mar 23, 2020 at 12:04:58PM -0700, Dwarf Discussion wrote: > On 3/23/20 6:28 AM, Robinson, Paul via Dwarf-Discuss wrote: > > > From: Dwarf-Discuss On Behalf > > > Of Adrian Prantl via Dwarf-Discuss > > > > On Mar 19, 2020, at 5:49 PM, Michael Eager via Dwarf-Discuss > > disc...@lists.dwarfstd.org> wrote: > > > > > > > > My reading of sections 2.5 & 2.6 is that you cannot have a DW_OP_piece > > > in an DWARF expression. > > > > > > > > > > I wonder if this is an intentional part of the design because of > > > ambiguity/correctness issues or is this just something that happens to > > > fall out of the way the text is worded? I can see how such a restriction > > > might simplify DWARF consumers, but it also seems like an arbitrary > > > restriction for which there may not be a technical reason. > > > > My intuition (clearly I wasn't there at the time) is that this is like > > a C expression being an rvalue (DWARF expression) or lvalue (location > > description). Values and locations aren't the same thing. > > It is somewhat an L-value vs R-value issue. > > You can craft a DWARF expression to extract a value (an R-value) from > arbitrary memory locations or registers (for example, using DW_OP_and, > DW_OP_sh?, etc.) and place it on the top of the stack. A DW_OP_piece > operator doesn't do this. (There might be value in an operator which > extracts a value from a composite location.) > > A location (an L-value) which includes potentially multiple register or > memory locations and multiple DW_OP_piece or DW_OP_bit_piece operations > can't be evaluated by a simple stack-based expression interpreter. > > The design is intentional, AFAIK, not accidental. > > I think that the description has become a bit less clear with the addition > of the Implicit Location Descriptions in Section 2.6.1.1.4, which do compute > values, rather than locations. Perhaps these should have been described in > Section 2.5 as parts of a DWARF expression, not as parts of a Location > Description. > > The description (and implementation) of DWARF expressions and locations are > somewhat muddled together. This can be seen in the first sentence of > Section 2.5: >DWARF expressions describe how to compute a value or specify a >location. > A clearer definition would specify that the DWARF expression only computes a > value, and leaving what that value means (e.g., register/memory contents, > arbitrary computation, memory address) to the context in which the > expression is used. A more precise definition of a location, especially a > composite location, would help. > > Part of this is historical, with the expression evaluation initially used to > compute a memory address and then extended to accommodate composite > locations and further extended to compute arbitrary values. These extensions > were in divergent directions, affecting both location composition and value > co
Re: [Dwarf-Discuss] Use of Location Description operations in DWARF Expressions?
On 3/23/20 6:28 AM, Robinson, Paul via Dwarf-Discuss wrote: From: Dwarf-Discuss On Behalf Of Adrian Prantl via Dwarf-Discuss On Mar 19, 2020, at 5:49 PM, Michael Eager via Dwarf-Discuss disc...@lists.dwarfstd.org> wrote: My reading of sections 2.5 & 2.6 is that you cannot have a DW_OP_piece in an DWARF expression. I wonder if this is an intentional part of the design because of ambiguity/correctness issues or is this just something that happens to fall out of the way the text is worded? I can see how such a restriction might simplify DWARF consumers, but it also seems like an arbitrary restriction for which there may not be a technical reason. My intuition (clearly I wasn't there at the time) is that this is like a C expression being an rvalue (DWARF expression) or lvalue (location description). Values and locations aren't the same thing. It is somewhat an L-value vs R-value issue. You can craft a DWARF expression to extract a value (an R-value) from arbitrary memory locations or registers (for example, using DW_OP_and, DW_OP_sh?, etc.) and place it on the top of the stack. A DW_OP_piece operator doesn't do this. (There might be value in an operator which extracts a value from a composite location.) A location (an L-value) which includes potentially multiple register or memory locations and multiple DW_OP_piece or DW_OP_bit_piece operations can't be evaluated by a simple stack-based expression interpreter. The design is intentional, AFAIK, not accidental. I think that the description has become a bit less clear with the addition of the Implicit Location Descriptions in Section 2.6.1.1.4, which do compute values, rather than locations. Perhaps these should have been described in Section 2.5 as parts of a DWARF expression, not as parts of a Location Description. The description (and implementation) of DWARF expressions and locations are somewhat muddled together. This can be seen in the first sentence of Section 2.5: DWARF expressions describe how to compute a value or specify a location. A clearer definition would specify that the DWARF expression only computes a value, and leaving what that value means (e.g., register/memory contents, arbitrary computation, memory address) to the context in which the expression is used. A more precise definition of a location, especially a composite location, would help. Part of this is historical, with the expression evaluation initially used to compute a memory address and then extended to accommodate composite locations and further extended to compute arbitrary values. These extensions were in divergent directions, affecting both location composition and value computation. It would have been better if the two concepts were clearly distinguished. -- Michael Eager ___ Dwarf-Discuss mailing list Dwarf-Discuss@lists.dwarfstd.org http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
Re: [Dwarf-Discuss] Use of Location Description operations in DWARF Expressions?
> -Original Message- > From: Dwarf-Discuss On Behalf > Of Adrian Prantl via Dwarf-Discuss > Sent: Friday, March 20, 2020 1:29 PM > To: Michael Eager > Cc: dwarf-discuss@lists.dwarfstd.org > Subject: Re: [Dwarf-Discuss] Use of Location Description operations in > DWARF Expressions? > > > > > On Mar 19, 2020, at 5:49 PM, Michael Eager via Dwarf-Discuss disc...@lists.dwarfstd.org> wrote: > > > > My reading of sections 2.5 & 2.6 is that you cannot have a DW_OP_piece > in an DWARF expression. > > > > I wonder if this is an intentional part of the design because of > ambiguity/correctness issues or is this just something that happens to > fall out of the way the text is worded? I can see how such a restriction > might simplify DWARF consumers, but it also seems like an arbitrary > restriction for which there may not be a technical reason. My intuition (clearly I wasn't there at the time) is that this is like a C expression being an rvalue (DWARF expression) or lvalue (location description). Values and locations aren't the same thing. --paulr > That distinction is important, because if there is a *technical* reason > for not supporting them we should refrain from implementing this in LLVM. > But if there isn't, there is no harm done in implementing it as an > extension, and DWARF consumers that don't support it can just ignore these > expressions and return N/A. > > -- adrian > ___ > Dwarf-Discuss mailing list > Dwarf-Discuss@lists.dwarfstd.org > http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org ___ Dwarf-Discuss mailing list Dwarf-Discuss@lists.dwarfstd.org http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
Re: [Dwarf-Discuss] Use of Location Description operations in DWARF Expressions?
> On Mar 19, 2020, at 5:49 PM, Michael Eager via Dwarf-Discuss > wrote: > > My reading of sections 2.5 & 2.6 is that you cannot have a DW_OP_piece in an > DWARF expression. > I wonder if this is an intentional part of the design because of ambiguity/correctness issues or is this just something that happens to fall out of the way the text is worded? I can see how such a restriction might simplify DWARF consumers, but it also seems like an arbitrary restriction for which there may not be a technical reason. That distinction is important, because if there is a *technical* reason for not supporting them we should refrain from implementing this in LLVM. But if there isn't, there is no harm done in implementing it as an extension, and DWARF consumers that don't support it can just ignore these expressions and return N/A. -- adrian ___ Dwarf-Discuss mailing list Dwarf-Discuss@lists.dwarfstd.org http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
Re: [Dwarf-Discuss] Use of Location Description operations in DWARF Expressions?
On 3/19/20 9:30 AM, David Stenberg via Dwarf-Discuss wrote: Hi! This is something that has popped up in a number of LLVM patch reviews, and is something that we would like to get some help with clarifying. For DWARF[345], may a DWARF Expression (described in section 2.5) contain any of the operations listed under the Location Descriptions section (2.6)? For example, may a DW_AT_call_value in DWARF5, which "is a DWARF expression", contain any of those operations (e.g. a DW_OP_piece)? DWARF5 section 2.5 specifies the following for the operations under section 2.6: "In addition to the general operations that are defined here, operations that are specific to location descriptions are defined in Section 2.6 on page 38." Reading section 2.5 and 2.6, I have interpreted the standard as if DWARF Expressions and Location Descriptions are disparate things; Location Descriptions may describe locations _using_ DWARF Expressions, but in places where a DWARF Expression is expected, you may not have a Location Description in its place. Is that correct? Given the above quote, and if my interpretation of how DWARF Expressions and Location Descriptions correlate is correct, I would assume that the answer to this mail's initial question is no. Or am I overlooking somethere here? My reading of sections 2.5 & 2.6 is that you cannot have a DW_OP_piece in an DWARF expression. A DWARF expression, when evaluated (as the result of evaluating a DW_AT_call_value), yields a value which is at the top of the stack. A Composite Location Description, containing DW_OP_piece, as mentioned on page 42, "does not compute a value or store any result on the DWARF stack." The DW_OP_reg* operations in a Location Description name a register, but do not modify the stack. (See comment page 40.) Implicit Location Descriptions may only appear in Location Descriptions. The wording of the text says that the three operations specify a value, but do not say that this value is pushed on the stack. -- Michael Eagerea...@eagercon.com 1960 Park Blvd., Palo Alto, CA 94306 ___ Dwarf-Discuss mailing list Dwarf-Discuss@lists.dwarfstd.org http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
[Dwarf-Discuss] Use of Location Description operations in DWARF Expressions?
Hi! This is something that has popped up in a number of LLVM patch reviews, and is something that we would like to get some help with clarifying. For DWARF[345], may a DWARF Expression (described in section 2.5) contain any of the operations listed under the Location Descriptions section (2.6)? For example, may a DW_AT_call_value in DWARF5, which "is a DWARF expression", contain any of those operations (e.g. a DW_OP_piece)? DWARF5 section 2.5 specifies the following for the operations under section 2.6: "In addition to the general operations that are defined here, operations that are specific to location descriptions are defined in Section 2.6 on page 38." Reading section 2.5 and 2.6, I have interpreted the standard as if DWARF Expressions and Location Descriptions are disparate things; Location Descriptions may describe locations _using_ DWARF Expressions, but in places where a DWARF Expression is expected, you may not have a Location Description in its place. Is that correct? Given the above quote, and if my interpretation of how DWARF Expressions and Location Descriptions correlate is correct, I would assume that the answer to this mail's initial question is no. Or am I overlooking somethere here? Best regards, David ___ Dwarf-Discuss mailing list Dwarf-Discuss@lists.dwarfstd.org http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
