<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">Hi Leonard,<div class=""><br class=""></div><div class="">I haven’t looked at your patch in detail, but I think that this is a step in the right direction. I would like to see new “Constant*”’s that directly map onto the relocations that various object formats use (for example, macho has a relocation for “&g1-&g2+cst”). This would get us to a more principled lowering in many cases as well as make the backend modeling more specific.</div><div class=""><br class=""></div><div class="">-Chris<br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Aug 20, 2020, at 11:29 AM, Leonard Chan via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><span id="gmail-docs-internal-guid-bcd6a4a8-7fff-6db4-f093-d40e55405145" class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 11pt; font-family: Arial; background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">Hi all,</span></div><br class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 11pt; font-family: Arial; background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">We would like to propose a new Constant type in LLVM for representing entries in the Procedure Linkage Table (PLT).</span></div><br class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 11pt; font-family: Arial; background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">The PLT is a data structure used for dispatching position-independent function calls to appropriate functions where the address of the function is not known statically. Right now, if a call is made to a function, it may be lowered to a direct call to the function itself or the PLT entry for that function. LLVM has checks that dictate if the function call should be a direct reference or PLT entry, but we don’t have a way in IR to semantically represent that the PLT entry should be used.</span></div><br class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 11pt; font-family: Arial; background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">The proposed constant would be analogous to BlockAddress, but instead represent the PLT entry for functions. The usage could look something like:</span></div><br class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 11pt; font-family: Inconsolata, monospace; background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">pltentry(@function)</span></div><br class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 11pt; font-family: Arial; background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">and would always have the same type as the function. A </span><span style="font-size: 11pt; font-family: Inconsolata, monospace; background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">pltentry</span><span style="font-size: 11pt; font-family: Arial; background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class=""> would operate exactly like a function, but the main difference is that it’s lowered to the PLT entry (</span><span style="font-size: 11pt; font-family: Inconsolata, monospace; background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">function@plt</span><span style="font-size: 11pt; font-family: Arial; background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">) on targets that support PLT relocations. The linker can then decide if it should be relaxed into a direct reference or remain a PLT entry.</span></div><br class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 11pt; font-family: Arial; background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">I have a very rough WIP implementation at </span><a href="https://reviews.llvm.org/D77248" style="text-decoration-line:none" class=""><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-variant-numeric:normal;font-variant-east-asian:normal;text-decoration-line:underline;vertical-align:baseline;white-space:pre-wrap" class="">https://reviews.llvm.org/D77248</span></a><span style="font-size: 11pt; font-family: Arial; background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">.</span></div><br class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 11pt; font-family: Arial; background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">Thoughts and opinions?</span></div><br class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 11pt; font-family: Arial; background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">Thanks,</span></div><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 11pt; font-family: Arial; background-color: transparent; font-variant-numeric: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">Leonard</span></div></span><br class="gmail-Apple-interchange-newline"></div>
_______________________________________________<br class="">LLVM Developers mailing list<br class=""><a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a><br class="">https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev<br class=""></div></blockquote></div><br class=""></div></body></html>