<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Mar 10, 2015, at 4:35 PM, David Blaikie <<a href="mailto:dblaikie@gmail.com" class="">dblaikie@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><br class=""><div class="gmail_extra"><br class=""><div class="gmail_quote">On Tue, Mar 10, 2015 at 4:28 PM, Robinson, Paul <span dir="ltr" class=""><<a href="mailto:Paul_Robinson@playstation.sony.com" target="_blank" class="">Paul_Robinson@playstation.sony.com</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<div class=""><span class=""><p class="MsoNormal" style="margin-left:.5in">Yes this is exactly the edge case that we were relying on up to now. There are two problems I have with that:<u class=""></u><u class=""></u></p><p class="MsoNormal" style="margin-left:.5in">a) how do we implement the “defined by the ABI” predicate correctly? Assume that it’s always the subregister at offset 0 and wait until someone complains?<u class=""></u><u class=""></u></p><p class="MsoNormal" style="margin-left:4.8pt"><br class="">
I'd probably be OK with this, open to other opinions, but it seems pretty simply like "we stuffed this number in a big register, but we don't need all the bits in the big register, so take the lowest bits that we specify" - any platform that did anything stranger
than that... well, we should probably have a talk with/about them anyway, so I wouldn't mind if them running into this feature was what caused us to have that discussion.<br class="">
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</span><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d" class="">You mean like X86 vector registers? They're implicitly arrays/structs of values of a smaller size. I don't know that the value of interest always ends up in
the "low" part, but maybe it does. I really don't have a handle on X86 vector instructions.</span></p></div></div></blockquote><div class=""><br class="">Can't say I know a great deal about it - but sure, if we can concoct a test case where part of a variable resides in part of a vector register, it'd be worth checking that it behaves appropriately.<br class=""></div></div></div></div></div></blockquote><div><br class=""></div><div>I’m not worried so much about the case were the value of interest is in one of the upper parts, because that will not trigger the questionable edge-case; it would translate into a shift-right and mask operation. The real question (for me) is what “defined by the ABI” is ever supposed to mean. I suppose an ABI may specify that a vector of 8-bit values be passed in the subregisters v0-v15 of a 128-bit register w and then we could encode a 2-byte vector that is in v0+v1 as DW_OP_reg[w] DW_OP_piece 1 DW_OP_reg[w] DW_OP_piece 1, but even that I find dubious, because of the amount of implicit state necessary to evaluate this expression. And a single byte in v1 still would need to be expressed via shift+mask.</div><div><br class=""></div><div>-- adrian</div><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class="gmail_extra"><div class="gmail_quote"><div class=""> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div lang="EN-US" link="blue" vlink="purple" class=""><div class=""><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d" class=""><u class=""></u><u class=""></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d" class="">--paulr<u class=""></u><u class=""></u></span></p><p class="MsoNormal"><a name="14c060454ece94a0__MailEndCompose" class=""><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d" class=""><u class=""></u> <u class=""></u></span></a></p>
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<div style="border:none;border-top:solid #b5c4df 1.0pt;padding:3.0pt 0in 0in 0in" class=""><p class="MsoNormal"><b class=""><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"" class="">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"" class=""> <a href="mailto:llvm-commits-bounces@cs.uiuc.edu" target="_blank" class="">llvm-commits-bounces@cs.uiuc.edu</a> [mailto:<a href="mailto:llvm-commits-bounces@cs.uiuc.edu" target="_blank" class="">llvm-commits-bounces@cs.uiuc.edu</a>]
<b class="">On Behalf Of </b>David Blaikie<br class="">
<b class="">Sent:</b> Tuesday, March 10, 2015 3:36 PM<br class="">
<b class="">To:</b> Adrian Prantl<br class="">
<b class="">Cc:</b> <a href="mailto:reviews%2BD8233%2Bpublic%2Bf5c9e8745840f37b@reviews.llvm.org" target="_blank" class="">reviews+D8233+public+f5c9e8745840f37b@reviews.llvm.org</a>; <a href="mailto:llvm-commits@cs.uiuc.edu" target="_blank" class="">llvm-commits@cs.uiuc.edu</a><br class="">
<b class="">Subject:</b> Re: [PATCH] Fix the remainder of PR22762 (GDB is crashing on DW_OP_piece being used inside of DW_AT_frame_base)<u class=""></u><u class=""></u></span></p>
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<div class=""><p class="MsoNormal">On Tue, Mar 10, 2015 at 3:26 PM, Adrian Prantl <<a href="mailto:aprantl@apple.com" target="_blank" class="">aprantl@apple.com</a>> wrote:<u class=""></u><u class=""></u></p>
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<div class=""><p class="MsoNormal">On Mar 10, 2015, at 3:14 PM, David Blaikie <<a href="mailto:dblaikie@gmail.com" target="_blank" class="">dblaikie@gmail.com</a>> wrote:<u class=""></u><u class=""></u></p>
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<div class=""><p class="MsoNormal">On Tue, Mar 10, 2015 at 2:50 PM, Adrian Prantl <<a href="mailto:aprantl@apple.com" target="_blank" class="">aprantl@apple.com</a>> wrote:<u class=""></u><u class=""></u></p><p class="MsoNormal">Hi echristo, dblaikie,<br class="">
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<a href="http://llvm.org/bugs/show_bug.cgi?id=22762" target="_blank" class="">http://llvm.org/bugs/show_bug.cgi?id=22762</a><br class="">
The symptom was that DW_AT_frame_base should never use a DW_OP_(bit)_piece, the bug was that AddMachineRegPiece incorrectly created pieces to describe values that occupy only a subregister. Change this to emit a bit mask instead.<br class="">
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I'm posting this for review because emitting the bit mask increases the size occupied for DWARF expressions for sub-registers (~5 bytes for a 32-bit subregister). Previously we would (incorrectly!) use DW_OP_piece to describe a value occupying part of a register.
However, "DW_OP_piece provides a way of describing how large a part of a variable a particular DWARF location description refers to.",<u class=""></u><u class=""></u></p>
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The spec also says "If the piece is located in a register, but does not occupy the entire register, the placement of the piece within that register is defined by the ABI. " - so we can use this in some cases at least. Should we? I assume it just means if we
say _piece of size 1 in a register of size 4 we get the low byte (whatever definition of 'low' there is)?<br class="">
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<blockquote style="border:none;border-left:solid #cccccc 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in" class=""><p class="MsoNormal">not the size and offset of an entire variable inside a super-register. The way that most debuggers implement DW_OP_piece this sort of works out for subregisters that are at offset 0, but it causes confusion if the expression needs to be
composed (such as in DW_AT_frame_base, or if the subregister contains only a part of the variable).<u class=""></u><u class=""></u></p>
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<div class=""><p class="MsoNormal">Yes this is exactly the edge case that we were relying on up to now. There are two problems I have with that:<u class=""></u><u class=""></u></p>
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<div class=""><p class="MsoNormal">a) how do we implement the “defined by the ABI” predicate correctly? Assume that it’s always the subregister at offset 0 and wait until someone complains?<u class=""></u><u class=""></u></p>
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I'd probably be OK with this, open to other opinions, but it seems pretty simply like "we stuffed this number in a big register, but we don't need all the bits in the big register, so take the lowest bits that we specify" - any platform that did anything stranger
than that... well, we should probably have a talk with/about them anyway, so I wouldn't mind if them running into this feature was what caused us to have that discussion.<br class="">
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<div class=""><p class="MsoNormal">b) it doesn’t compose well, so we’d need to explicitly forbid it inside of DW_AT_frame_base and inside of a larger piece expression.<u class=""></u><u class=""></u></p>
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This bit I don't really understand - perhaps you could provide some expression examples?<br class="">
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<div class=""><p class="MsoNormal">b) is doable, if ugly, but I’d need some help with a).<u class=""></u><u class=""></u></p>
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<div class=""><p class="MsoNormal"><span style="color:#888888" class="">-- adrian<u class=""></u><u class=""></u></span></p>
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