From 147efc3421ac3860720863e796406b3918e7c1c7 Mon Sep 17 00:00:00 2001 From: "H.J. Lu" Date: Tue, 3 Mar 2015 11:38:27 -0800 Subject: [PATCH 2/3] Combine GOTPLT and GOT Slots In the small and medium models, when there are both PLT and GOT references to the same function symbol, normally linker creates a GOTPLT slot for PLT entry and a GOT slot for GOT reference. A run-time JUMP_SLOT relocation is created to update the GOTPLT slot and a run-time GLOB_DAT relocation is created to update the GOT slot. Both JUMP_SLOT and GLOB_DAT relocations apply the same symbol value to GOTPLT and GOT slots, respectively, at run-time. As an optimization, linker may combine GOTPLT and GOT slots into a single GOT slot and remove the run-time JUMP_SLOT relocation. * Makefile (INCLUDES): Add linker-optimization.tex. * abi.tex: Incldue linker-optimization. * linker-optimization.tex: New file. --- Makefile | 2 +- abi.tex | 1 + linker-optimization.tex | 63 +++++++++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 65 insertions(+), 1 deletion(-) create mode 100644 linker-optimization.tex diff --git a/Makefile b/Makefile index 7cb148a..89dfd18 100644 --- a/Makefile +++ b/Makefile @@ -1,7 +1,7 @@ INCLUDES := execution.tex low-level-sys-info.tex development.tex \ introduction.tex sw-installation.tex dl.tex libraries.tex \ macros.tex conventions.tex abi-index.tex object-files.tex \ - kernel.tex fortran.tex + kernel.tex fortran.tex linker-optimization.tex ALL_FILES := abi.tex $(INCLUDES) diff --git a/abi.tex b/abi.tex index f4c3f9f..fc025f0 100644 --- a/abi.tex +++ b/abi.tex @@ -108,6 +108,7 @@ Draft Version \version}} \appendix \include{kernel} +\include{linker-optimization} \include{abi-index} \end{document} diff --git a/linker-optimization.tex b/linker-optimization.tex new file mode 100644 index 0000000..c34afd5 --- /dev/null +++ b/linker-optimization.tex @@ -0,0 +1,63 @@ +\chapter{Linker Optimization} + +This chapter describes optimizations which may be performed by linker. + +\section{Combine GOTPLT and GOT Slots} +In the small and medium models, when there are both PLT and GOT references +to the same function symbol, normally linker creates a GOTPLT slot for PLT +entry and a GOT slot for GOT reference. A run-time JUMP_SLOT relocation is +created to update the GOTPLT slot and a run-time GLOB_DAT relocation is +created to update the GOT slot. Both JUMP_SLOT and GLOB_DAT relocations +apply the same symbol value to GOTPLT and GOT slots, respectively, at +run-time. + +As an optimization, linker may combine GOTPLT and GOT slots into a single +GOT slot and remove the run-time JUMP_SLOT relocation. It replaces the +regular PLT entry: + +\begin{figure}[H] +\Hrule +\caption{Procedure Linkage Table Entry Via GOTPLT Slot} +\label{gotplt_plt} +\begin{footnotesize} +\begin{verbatim} + .PLT: jmp [GOTPLT slot] + pushq relocation index + jmp .PLT0 +\end{verbatim} +\end{footnotesize} +\Hrule +\end{figure} + +\noindent +with an GOT PLT entry with an indirect jump via the GOT slot: +\indent + +\begin{figure}[H] +\Hrule +\caption{Procedure Linkage Table Entry Via GOT Slot} +\label{got_plt} +\begin{footnotesize} +\begin{verbatim} + .PLT: jmp [GOT slot] + nop +\end{verbatim} +\end{footnotesize} +\Hrule +\end{figure} + +\noindent +and resolves the PLT reference to the GOT PLT entry. Indirect \code{jmp} +is an 5-byte instruction. \code{nop} can be encoded as a 3-byte +instruction or a 11-byte instruction for 8-byte or 16-byte PLT slot. +A separate PLT with 8-byte slots may be used for this optimization. +\indent + +This optimization isn't applicable to the \texttt{STT_GNU_IFUNC} symbols +since their GOTPLT slots are resolved to the selected implementation and +their GOT slots are resolved to their PLT entries. + +This optimization must be avoided if pointer equality is needed since +the symbol value won't be cleared in this case and the dynamic linker +won't update the GOT slot. Otherwise, the resulting binary will get +into an infinite loop at run-time. -- 2.4.3