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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Hi Igor,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">> </span>It sounds like the linker has to be aware of the .eh_frame section details to be able to generate .eh_frame_hdr and eliminate duplicate CIEs, right?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Yes, a linker needs some details but not all of them. It needs to know sizes of records and initial locations (PC Begin) to find out which functions FDEs belong to<span style="mso-fareast-language:EN-US">.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">> </span>So, is there any difference whether it knows that in one place or two?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">What do you mean “one place or two”? If .eh_frame_hdr is not created a linker does not need to parse .eh_frame sections. It simply merges them into one section. The format of .eh_frame allows to do this without parsing .eh_frame sections.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal">Evgeny Astigeevich<span style="mso-fareast-language:EN-US"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:12.0pt;color:black">From: </span></b><span style="font-size:12.0pt;color:black">Igor Kudrin <ikudrin.dev@gmail.com><br>
<b>Date: </b>Thursday, 9 November 2017 at 11:29<br>
<b>To: </b>Rui Ueyama <ruiu@google.com>, Evgeny Astigeevich <Evgeny.Astigeevich@arm.com><br>
<b>Cc: </b>"llvm-dev@lists.llvm.org" <llvm-dev@lists.llvm.org>, nd <nd@arm.com><br>
<b>Subject: </b>Re: [llvm-dev] [RFC] Making .eh_frame more linker-friendly<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p><a name="_MailOriginalBody">It sounds like the linker has to be aware of the .eh_frame section details to be able to generate .eh_frame_hdr and eliminate duplicate CIEs, right?<o:p></o:p></a></p>
<p><span style="mso-bookmark:_MailOriginalBody">So, is there any difference whether it knows that in one place or two?<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="mso-bookmark:_MailOriginalBody">Best Regards,<br>
Igor Kudrin<br>
C++ Developer, Access Softek, Inc.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="mso-bookmark:_MailOriginalBody">On 27-Oct-17 3:43, Rui Ueyama via llvm-dev wrote:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="mso-bookmark:_MailOriginalBody">On Thu, Oct 26, 2017 at 1:13 PM, Evgeny Astigeevich <</span><a href="mailto:Evgeny.Astigeevich@arm.com" target="_blank"><span style="mso-bookmark:_MailOriginalBody">Evgeny.Astigeevich@arm.com</span><span style="mso-bookmark:_MailOriginalBody"></span></a><span style="mso-bookmark:_MailOriginalBody">>
wrote:<o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="mso-bookmark:_MailOriginalBody">Hi,<o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="mso-bookmark:_MailOriginalBody"> <o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="mso-bookmark:_MailOriginalBody">There will be problems with eh_frame_hdr. Eh_frame_hdr is needed to use the binary search instead of the linear search. Having eh_frame
per a function will cause no eh_frame_hdr or multiple eh_frame_hdr and will degrade search from binary to linear.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="mso-bookmark:_MailOriginalBody"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="mso-bookmark:_MailOriginalBody">Linkers would combine .eh_frame sections into one .eh_frame, so that's not an issue, no?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="mso-bookmark:_MailOriginalBody"> <o:p></o:p></span></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="mso-bookmark:_MailOriginalBody">As we create eh_frame_hdr in most cases there is no problem to filter out garbage eh_frame sections. If there is information about unused
symbols, the implementation is very simple. BTW there is no need to do full decoding of eh_frame records to remove garbage.<o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="mso-bookmark:_MailOriginalBody">Paul is right there will be code size overhead. Eh_frame is usually created per a compilation module with common information in CFI.
Multiple eh_frames will cause a lot of redundant CFI. There might be a case when the total size of redundant CFIs will be greater than the total size of removed garbage.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="mso-bookmark:_MailOriginalBody"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="mso-bookmark:_MailOriginalBody">As I wrote in the previous message, I don't think there's a size issue in link results because even existing linkers merge CIEs by contents.<o:p></o:p></span></p>
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