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Generating ancilliary sections with gas (fwd)

by Nicolas Pitre

Hello Michael, We do have more and more instances of the following issues turning up in the kernel requiring toolchain assistance to solve the problem properly. Could you or someone from your team follow this up please? ---------- Forwarded message ---------- Date: Tue, 1 Feb 2011 12:16:48 +0000 From: Dave Martin <dave.martin(a)linaro.org> To: binutils(a)sourceware.org Cc: linaro-toolchain <linaro-toolchain(a)lists.linaro.org> Subject: Generating ancilliary sections with gas Hi all, Every now and again I come across a situation where it would be really useful to be able to query the assembler state during assembly: for example, to query and do something based on the current section name. This makes it possible to write generic macros to do certain things which otherwise require manual maintenance, or complex and fragile external preprocessing. Below, I give a real-world example of the problem, and sketch out a possible solution. What do people think of this approach? Does anyone have any better ideas on how to solve this? Cheers ---Dave EXAMPLE An example is the generation of custom ancilliary sections. Suppose you want to write macros which record fixup information. Currently, there's no way to put each fixup in an appropriately named section automatically within gas. Tellingly, gas has had to grow the ability to do this internally at least for ARM, since the exception handling information in .ARM.ex{idx,tab}* must go in sections with names based on the associated section name. However, this ancillary section generation support is neither flexible nor exposed to the user. By putting fixups in sections whose names are based on the name of the section they refer to, selective link-time discard of the fixups (and hence the code referenced by the fixups) will work; otherwise it doesn't. This would help avoid a situation where we have to keep dead code in the kernel because custom fixups are applied to it: at run-time, the code gets fixed up, then is thrown away. The fixups can't be selectively discarded because they are all in the same section: we seem have to no good way to separate them out into separate sections appropriately. For context, see: http://www.spinics.net/lists/arm-kernel/msg112268.html PROPOSAL To solve the problem of generating custom ancillary sections during assembly, here's a simple proposal: introducing a new kind of macro argument can make aspects of the assembler state available to macros in a flexible way, with only minimal implementation required. Basically, the macro qualifier field could be used to identify arguments which are filled in by the assembler with information about the assembly state, rather than being filled in by the invoker of the macro: e.g.: .macro mymacro name:req, flags, secname:current_section /* ... */ .pushsection "\secname\name", "\flags" /* ... */ .popsection .endm /* ... */ mymacro .ancillary, "a" During expansion, \name and \flags are expanded as normal. But \secname is substituted instead with the current section name, so the macro expansion would look like this: /* ... */ .pushsection ".text.ancillary", "a" /* ... */ .popsection Without the special :current_section argument, it doesn't appear possible to implement a macro such as mymacro in a generic way. This surely isn't the only way to achieve the goal, and it's probably not the best way, but it does have some desirable features. Principally, while a new pseudo-op(s) could have been defined to append text to the current section name, etc., allowing the current section name to be appear as a macro parameter avoids prejudicing the way the text is used. So there should never be a need to introduce additional pseudo-ops to do things with the current section name: with this patch, the user can always implement their own macro to do the desired thing. This gets the desired behaviour and maximum flexibility, while keeping the implementation in gas very simple. Also, using the macro expansion system in this way allows the caller a free choice of macro parameter names, and so pretty much guarantees that existing code won't get broken by the change. Because my hack is currently simplistic, it has shortcomings: in particular, it's not desirable to parse an argument from the invocation line at all to fill a :current_section argument. Currently, an argument is read in if present, but its value is ignored and the current section name pasted in at macro expansion time instead. However, that should be straightforward to fix with a bit more code. Of course, there's no reason only to expose the current section name in this way. Any aspect of the the assembler state (current subsection, current section flags, current instruction set, current macro mode, etc.) could be made available in a similar way. USAGE EXAMPLE AND PATCH Note that the specific implementation described here is intended to be illustrative, rather than complete or final. binutils$ cat <<EOF >tst.s .macro push_ancillary_section name:req, flags, csec:current_section .pushsection "\name\csec", "\flags" .endm .macro register_fixup _register_fixup 100\@ .endm .macro _register_fixup label:req \label : push_ancillary_section .fixup, "a" .long \label\(b) .popsection .endm .long 1 register_fixup .long 2 .data .long 3 register_fixup .long 4 .long 5 register_fixup .long 6 EOF binutils$ gas/as-new -ahlms -o tst.o tst.s ARM GAS tst.s page 1 1 .macro push_ancillary_section name:req, flags, csec:current_section 2 .pushsection "\name\csec", "\flags" 3 .endm 4 5 .macro register_fixup 6 _register_fixup 100\@ 7 .endm 8 9 .macro _register_fixup label:req 10 \label : 11 push_ancillary_section .fixup, "a" 12 .long \label\(b) 13 .popsection 14 .endm 15 16 0000 01000000 .long 1 17 register_fixup 17 > _register_fixup 1000 17 >> 1000: 17 >> push_ancillary_section .fixup,"a" 17 >>> .pushsection ".fixup.text","a" 17 0000 04000000 >> .long 1000b 17 >> .popsection 18 0004 02000000 .long 2 19 20 .data 21 0000 03000000 .long 3 22 register_fixup 22 > _register_fixup 1003 22 >> 1003: 22 >> push_ancillary_section .fixup,"a" 22 >>> .pushsection ".fixup.data","a" 22 0000 04000000 >> .long 1003b 22 >> .popsection 23 0004 04000000 .long 4 24 0008 05000000 .long 5 25 register_fixup 25 > _register_fixup 1006 25 >> 1006: 25 >> push_ancillary_section .fixup,"a" 25 >>> .pushsection ".fixup.data","a" 25 0004 0C000000 >> .long 1006b 25 >> .popsection 26 000c 06000000 .long 6 ARM GAS tst.s page 2 NO DEFINED SYMBOLS NO UNDEFINED SYMBOLS binutils$ arm-linux-gnueabi-objdump -rs tst.o tst.o: file format elf32-littlearm RELOCATION RECORDS FOR [.fixup.text]: OFFSET TYPE VALUE 00000000 R_ARM_ABS32 .text RELOCATION RECORDS FOR [.fixup.data]: OFFSET TYPE VALUE 00000000 R_ARM_ABS32 .data 00000004 R_ARM_ABS32 .data Contents of section .text: 0000 01000000 02000000 ........ Contents of section .data: 0000 03000000 04000000 05000000 06000000 ................ Contents of section .fixup.text: 0000 04000000 .... Contents of section .fixup.data: 0000 04000000 0c000000 ........ Contents of section .ARM.attributes: 0000 41150000 00616561 62690001 0b000000 A....aeabi...... 0010 08010901 2c01 ....,. diff --git a/gas/macro.c b/gas/macro.c index e392883..95c4de1 100644 --- a/gas/macro.c +++ b/gas/macro.c @@ -516,6 +516,8 @@ do_formals (macro_entry *macro, int idx, sb *in) formal->type = FORMAL_REQUIRED; else if (strcmp (qual.ptr, "vararg") == 0) formal->type = FORMAL_VARARG; + else if (strcmp (qual.ptr, "current_section") == 0) + formal->type = FORMAL_CURRENT_SECTION; else as_bad_where (macro->file, macro->line, @@ -540,6 +542,15 @@ do_formals (macro_entry *macro, int idx, sb *in) name, macro->name); } + else if (formal->type == FORMAL_CURRENT_SECTION) + { + sb_reset (&formal->def); + as_warn_where (macro->file, + macro->line, + _("Pointless default value for current_section parameter `%s' in macro `%s'"), + name, + macro->name); + } } /* Add to macro's hash table. */ @@ -734,7 +745,11 @@ sub_actual (int start, sb *in, sb *t, struct hash_control *formal_hash, ptr = (formal_entry *) hash_find (formal_hash, sb_terminate (t)); if (ptr) { - if (ptr->actual.len) + if (ptr->type == FORMAL_CURRENT_SECTION) + { + sb_add_string (out, segment_name (now_seg)); + } + else if (ptr->actual.len) { sb_add_sb (out, &ptr->actual); } diff --git a/gas/macro.h b/gas/macro.h index edc1b6b..ea6cabb 100644 --- a/gas/macro.h +++ b/gas/macro.h @@ -38,7 +38,8 @@ enum formal_type { FORMAL_OPTIONAL, FORMAL_REQUIRED, - FORMAL_VARARG + FORMAL_VARARG, + FORMAL_CURRENT_SECTION, }; /* Describe the formal arguments to a macro. */ _______________________________________________ linaro-toolchain mailing list linaro-toolchain(a)lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-toolchain

15 years, 3 months

[ACTIVITY] report week 27

by Peter Maydell

RAG: Red: Amber: OMAP3 patch upstreaming is slower progress than hoped Green: Current Milestones: || || Planned || Estimate || Actual || ||qemu-linaro-2011-07 || 2011-07-21 || 2011-07-21 || || Historical Milestones: ||qemu-linaro 2011-04 || 2011-04-21 || 2011-04-21 || 2011-04-21 || ||qemu-linaro 2011-05 || 2011-05-19 || 2011-05-19 || n/a || ||close out 1105 blueprints || 2011-05-28 || 2011-05-28 || 2011-05-19 || ||complete 1111 planning || 2011-05-28 || 2011-05-28 || 2011-05-27 || ||qemu-linaro-2011-06 || 2011-06-16 || 2011-06-16 || 2011-06-16 || == upstream-omap3-patches == * split and did most of the cleanup of 'overhaul onenand support' patch * updated the omap gpio qdev patchset in response to review comments, just about ready to send v2 * this is going more slowly than I had anticipated == other == * patch review, etc * confirmed attendance at KVM Forum and LinuxCon NA Current qemu patch status is tracked here: https://wiki.linaro.org/PeterMaydell/QemuPatchStatus Absences: 1-5 August: Linaro sprint 1111 15-19 August: KVM Forum and LinuxCon NA, Vancouver

15 years, 3 months

Updated armel cross toolchains for lucid/maverick/natty releases

by Marcin Juszkiewicz

Hi Linaro backport PPA [1] got updated to latest versions of armel cross toolchains -- oneiric packages were used as a base. What got changed: - gcc 4.4 was updated to 4.4.6-3ubuntu1 - gcc 4.5 was updated to 4.5.3-1ubuntu2 - binutils was updated to 2.21.52.20110606-1ubuntu1 - eglibc was updated to 2.13-6ubuntu2 - gcc 4.6 was provided as 4.6.0-14ubuntu1 in Maverick, Natty - gcc-defaults-armel-cross was updated to 1.6 in Maverick, Natty (uses gcc-4.6 as default) There is no gcc-4.6 for Lucid currently as it requires newer versions of few libraries (mpfr, mpc) and one of rule of this PPA is "do not update packages which may affect other packages". Please test them and report any bugs found. 1. https://launchpad.net/~linaro-maintainers/+archive/toolchain/

15 years, 3 months

[ACTIVITY] Jul 4 - Jul 8

by Ulrich Weigand

== GDB == * Posted patch to fix shared library remote test problems (#804387). * Started reviewing Yao's latest Thumb-2 displaced stepping patch. == GCC == * Reviewed and approved Richard's mainline reload patch to fix #803232 (ICE on code that uses vld4q_s16() NEON intrinsic). * Followed up on gcc-patches to address concerns about Julian's unaligned access patch. Mit freundlichen Gruessen / Best Regards Ulrich Weigand -- Dr. Ulrich Weigand | Phone: +49-7031/16-3727 STSM, GNU compiler and toolchain for Linux on System z and Cell/B.E. IBM Deutschland Research & Development GmbH Vorsitzender des Aufsichtsrats: Martin Jetter | Geschäftsführung: Dirk Wittkopp Sitz der Gesellschaft: Böblingen | Registergericht: Amtsgericht Stuttgart, HRB 243294

15 years, 3 months

[ACTIVITY] Weekly status

by Richard Sandiford

== This week == * Looked at why the fix for #721531 wasn't working on the Linaro branches. Wrote follow-up patches for both. Fix now committed to 4.5 and 4.6. * Looked at #736007. Submitted and committed patch upstream. * Looked at some "odd" ivopts behaviour. It turned out that this was working mostly as expected. I'm still wondering about a couple of tweaks. * Looked briefly at a miscompilation of vector code that turned out to occur during predictive commoning. I haven't yet checked whether the bug is there or not. For the time being, I'm using -fno-predictive-commoning so that I can get on with other stuff. * Looked at why the auto-inc-dec stuff wasn't as effective with current mainline. It turns out that the new misaligned load/store patterns are using overly weak predicates, so it appears to the RTL optimisers as though we support reg+offset addresses. * Reviewed the shrink-wrap patch. * More auto inc/dec. == Next week == * Backport the fix for #736007. * More loop stuff. Richard

15 years, 3 months

[ACTIVITY] 4th - 8th July

by Asa Sandahl

Achieved: * HW in place. Borrowed another Panda board that I can use permanently. * linaro-media-create installed and working. Flashed sd-card with 11.05 for Panda board. * Played around with the Panda board. Networking is on the way. I can ssh to my computer from the Panda but not the other way around yet. Issues: * Serial log from Panda board not working yet. Mike suggested I try a straight-through cable instead in of a null-modem cable. (Have not done this yet.) * The ST-E firewall (proxy) is causing me some headache. But at least everyone here is facing the same issues, so there are people to talk to about the problems. Good to know: * I will be on vacation 18th July - 15 August Best Regards Åsa

15 years, 3 months

[ACTIVITY] July 3-6

by Ira Rosen

Hi, - continued working on prevention of over-widening in vectorization - finalizing the patch - improvement of vectorizer peeling heuristic - merged to gcc-linaro-4.6 - vectorization of widen-mult with over-promoted operands - proposed for merge to gcc-linaro-4.6 - fixed PR 49610 - patch reviews I am on vacation tomorrow and on Sunday. Ira

15 years, 3 months

LLVM on ARM testing.

by Karel Gardas

Hello, I'm interested in LLVM correct performance on ARM hardware and it looks like LLVM is kind of sensitive to what GCC version is used for its compilation. I tested LLVM 2.9 as a reference point and LLVM HEAD as of June 29 on ARMv7 (two boards with two different Ubuntu versions) compiled by GCC 4.3.4, 4.4.1, 4.4.5, 4.5.2, 4.6.1 Linaro 2011.05 and 4.6.1 Linaro 2011.06. Please see http://ghcarm.wordpress.com/2011/07/03/llvm-on-arm-testing/ It looks like LLVM HEAD does have about 28 regressions in comparison with LLVM 2.9. But also Linaro's GCC 4.6.1s do have some regressions in comparison with older GCC 4.3.4 and 4.4.1. Also what is really interesting with LLVM is how much tests fails when compiled with -O2 or default -O3 compilation option. I don't know if the culprit here is LLVM code or just GCC miscompilation/overoptimization? Is there any testing I may do to help you fix those regressions? Thanks, Karel

15 years, 3 months

[ACTIVITY] 27th June - 1st July

by Andrew Stubbs

Worked on updates to the widening multiplies patches following upstream review. This was never going to be as easy as it first looked, so I've spent much of the week trying to figure out how make annoying type casts go away. Merged both FSF GCC 4.5 and 4.6 to Linaro GCC. Pushed the new tree to Launchpad for testing. ---- Upstream patched requiring review: * NEON scheduling patch http://gcc.gnu.org/ml/gcc-patches/2011-02/msg01431.html * Widening Multiplies 1/7 http://www.mail-archive.com/gcc-patches@gcc.gnu.org/msg08721.html * Widening Multiplies 2/7 http://www.mail-archive.com/gcc-patches@gcc.gnu.org/msg08723.html * Widening Multiplies 3/7 http://www.mail-archive.com/gcc-patches@gcc.gnu.org/msg08724.html * Widening Multiplies 4/7 http://www.mail-archive.com/gcc-patches@gcc.gnu.org/msg08725.html * Widening Multiplies 5/7 http://www.mail-archive.com/gcc-patches@gcc.gnu.org/msg08726.html * Widening Multiplies 6/7 http://www.mail-archive.com/gcc-patches@gcc.gnu.org/msg08727.html * Widening Multiplies 7/7 http://www.mail-archive.com/gcc-patches@gcc.gnu.org/msg08728.html

15 years, 3 months

[ACTIVITY] weekly status

by Ken Werner

Hi, * continued to look into how to add remote support for libunwind using ptrace * reworked the lookup of the ARM specific unwind tables for local unwinding * re-use the existent (dwarf related) infrastructure to find the ARM specific unwind tables rather than doing it on our own * removes some code and the limitation of only supporting a certain amount of unw tables * (hopefully) smooths the way for remote unwinding via ptrace * submitted a patch that fixes the syntax of inline assemblies for some GCC versions Regards Ken

15 years, 3 months

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