From: Florian Weimer <fw@deneb.enyo.de>
To: "Rahul Chaudhry via gnu-gabi" <gnu-gabi@sourceware.org>
Cc: Sriraman Tallam <tmsriram@google.com>,
Rahul Chaudhry <rahulchaudhry@google.com>,
hegdesmailbox@gmail.com, Florian Weimer <fweimer@redhat.com>,
David Edelsohn <dje.gcc@gmail.com>,
Rafael Avila de Espindola <rafael.espindola@gmail.com>,
Binutils Development <binutils@sourceware.org>,
Alan Modra <amodra@gmail.com>,
Cary Coutant <ccoutant@gmail.com>,
Xinliang David Li <davidxl@google.com>,
Sterling Augustine <saugustine@google.com>,
Paul Pluzhnikov <ppluzhnikov@google.com>,
Ian Lance Taylor <iant@google.com>,
"H.J. Lu" <hjl.tools@gmail.com>,
Luis Lozano <llozano@google.com>,
Peter Collingbourne <pcc@google.com>,
Rui Ueyama <ruiu@google.com>,
llvm-dev@lists.llvm.org
Subject: Re: Reducing code size of Position Independent Executables (PIE) by shrinking the size of dynamic relocations section
Date: Sun, 01 Jan 2017 00:00:00 -0000 [thread overview]
Message-ID: <874lozec25.fsf@mid.deneb.enyo.de> (raw)
In-Reply-To: <CAJRD=ooGubyUOLE6W7LHdeU2ZNDEG1A=84+P=1iOvfmD7-7GNg@mail.gmail.com> (Rahul Chaudhry via gnu-gabi's message of "Thu, 7 Dec 2017 14:51:20 -0800")
* Rahul Chaudhry via gnu-gabi:
> The encoding used is a simple combination of delta-encoding and a
> bitmap of offsets. The section consists of 64-bit entries: higher
> 8-bits contain delta since last offset, and lower 56-bits contain a
> bitmap for which words to apply the relocation to. This is best
> described by showing the code for decoding the section:
>
> typedef struct
> {
> Elf64_Xword r_data; /* jump and bitmap for relative relocations */
> } Elf64_Relrz;
>
> #define ELF64_R_JUMP(val) ((val) >> 56)
> #define ELF64_R_BITS(val) ((val) & 0xffffffffffffff)
>
> #ifdef DO_RELRZ
> {
> ElfW(Addr) offset = 0;
> for (; relative < end; ++relative)
> {
> ElfW(Addr) jump = ELFW(R_JUMP) (relative->r_data);
> ElfW(Addr) bits = ELFW(R_BITS) (relative->r_data);
> offset += jump * sizeof(ElfW(Addr));
> if (jump == 0)
> {
> ++relative;
> offset = relative->r_data;
> }
> ElfW(Addr) r_offset = offset;
> for (; bits != 0; bits >>= 1)
> {
> if ((bits&1) != 0)
> elf_machine_relrz_relative (l_addr, (void *) (l_addr + r_offset));
> r_offset += sizeof(ElfW(Addr));
> }
> }
> }
> #endif
That data-dependent “if ((bits&1) != 0)” branch looks a bit nasty.
Have you investigated whether some sort of RLE-style encoding would be
beneficial? If there are blocks of relative relocations, it might even
be possible to use vector instructions to process them (although more
than four relocations at a time are probably not achievable in a
power-efficient manner on current x86-64).
next prev parent reply other threads:[~2017-12-09 23:06 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CAGWvnynFwXFGLj3tAVgDatn0zmuHcWHyRNuDvR+wRZCXLnar_A@mail.gmail.com>
2017-01-01 0:00 ` Rafael Avila de Espindola
2017-01-01 0:00 ` David Edelsohn
2017-01-01 0:00 ` Suprateeka R Hegde
2017-01-01 0:00 ` Florian Weimer
2017-01-01 0:00 ` Suprateeka R Hegde
2017-01-01 0:00 ` Sriraman Tallam via gnu-gabi
2017-01-01 0:00 ` Rahul Chaudhry via gnu-gabi
2017-01-01 0:00 ` Ian Lance Taylor via gnu-gabi
2017-01-01 0:00 ` Cary Coutant
2017-01-01 0:00 ` Rahul Chaudhry via gnu-gabi
2017-01-01 0:00 ` Rahul Chaudhry via gnu-gabi
2017-01-01 0:00 ` Florian Weimer [this message]
2017-01-01 0:00 ` Sriraman Tallam via gnu-gabi
2017-01-01 0:00 ` Rahul Chaudhry via gnu-gabi
[not found] ` <CAORpzuMftCGpXUObOyoFY0=jorMBDWEDbQJ23DifTNW3v-WA6Q@mail.gmail.com>
2017-01-01 0:00 ` Rahul Chaudhry via gnu-gabi
2017-01-01 0:00 ` Cary Coutant
2017-01-01 0:00 ` Rahul Chaudhry via gnu-gabi
[not found] ` <CAORpzuPYsSBJtypm3NDcfcgRzos3WO4JjkvgiqpyBYBhoqLVFA@mail.gmail.com>
2018-01-01 0:00 ` Florian Weimer
2017-01-01 0:00 Sriraman Tallam
2017-01-01 0:00 ` Cary Coutant
2017-01-01 0:00 ` Markus Trippelsdorf
2017-01-01 0:00 ` Florian Weimer
2017-01-01 0:00 ` Alan Modra
2017-01-01 0:00 ` H.J. Lu
2017-01-01 0:00 ` Sriraman Tallam
2017-01-01 0:00 ` Rafael Espíndola
2017-01-01 0:00 ` Sriraman Tallam
2017-01-01 0:00 ` H.J. Lu
2017-01-01 0:00 ` Sriraman Tallam
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