gcc: if isPower64, add --with-long-double-64 --without-long-double-128 #170857

ghost · 2022-04-29T06:20:57Z

Description of changes

Only the final commit is unique to this PR.

Reviewers: pleases click "Commits" then click the last commit in order to see the diff. All the other commits are part of PRs which will (I would hope) merge before this one: #169378 (first five commits) and #170400 (second-to-last commit).

Description of changes

This commit causes powerpc*-*-gnu to be configured with 64-bit long double (the same as ordinary double) rather than the nonstandard, non-IEEE "IBM double" format from AIX.

This hopefully resolves a large number of package test failures caused by weird nonstandard floating-point semantics.

There is a rather long comment included in gcc/common/platform-flags.nix explaining the situation and the reasoning. I will move that into the commit message as this PR gets closer to mergeability.

Background

This is a continuation of the work in #170402, which I don't seem to be able to reopen even though I closed it (I guess you can only reopen self-closed bugs, not PRs?).

I think we have a working combination here, and pkgsStatic now interacts correctly with the rest of nixpkgs, the way it does on arm64 and x86, with no weird link failures. Still marked as draft while I finish rebuilding the world and use this for a while on my workstation.

Notes to be integrated into final commit message

The "fat binary" scheme is called a GNU IFUNC.
Here is where glibc says that it won't let you build it without AIX 128-bit long-doubles unless you also give up IEEE 128-bit long-doubles. To have only one long-double and have it be IEEE, glibc insists you use 64-bit long-doubles. I'm starting to understand why the musl people are so insistent about not wanting anything to do with this...

Things done

Built on platform(s)
- powerpc64le-linux
- x86_64-linux (in progress)
- aarch64-linux (in progress)
- mips64le-linux (in progress)
- x86_64-darwin
- aarch64-darwin
Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
Tested basic functionality of all binary files (usually in ./result/bin/)
Fits CONTRIBUTING.md.

As explained in the comment, this ensures that stage4-coreutils does not leak a reference to the bootstrap-files by way of libgmp. This will allow the next patch in this series to build stage4-coreutils using a dynamically-linked (rather than statically-linked) libgmp.

The usage of `makeStaticLibraries` in stdenv/linux/default.nix is prefaced by this comment: # Link GCC statically against GMP etc. This makes sense because # these builds of the libraries are only used by GCC, so it # reduces the size of the stdenv closure. However "these builds of the libraries are only used by GCC" is not actually true. As currently written, the stage4 coreutils links against these customized, static-ified libraries. Beside the fact that the code doesn't actually do what it says, this causes other problems as well. One example is #168983, which arises because have a dynamically-linked binary (coreutils) which is built from statically-linked libraries (libgmp.a); doing this causes mayhem on platforms where `-fstack-protector` needs an auxiliary `libssp.{so,a}` library; we end up with link failures because some parts of the resulting binary want `libssp.so` and other parts want `libssp_nonshared.a`. Let's make the code actually do what the comment says, by moving these definitions into the `gcc-unwrapped` override. This will cause the stage4-coreutils to link against libgmp dynamically, rather than statically. For this reason this commit depends on the previous commit, which allows that to be done without creating a forbidden reference from stdenv-final to the bootstrap-files.

During stdenv bootstrapping, coreutils is built twice. This makes troubleshooting very difficult, because both packages have name="coreutils", so it is a hassle to figure out "which coreutils am I using / is not building"? The first of these builds is used only in stage4, and is not part of the final stdenv. Let's label that one with a different `name` attribute to make it obvious which is which.

Co-authored-by: sternenseemann <sternenseemann@systemli.org>

A bug in libgcc which manifests when building static binaries using musl-libc was fixed in gcc 11.1.0, but the fix has not been backported to gcc 10. This commit cherry-picks the commit from upstream and applies it in the case (isPower64 && isMusl) where it matters: https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=cda41ce0e8414aec59e6b9fbe645d96e6e8193e2

This commit causes powerpc*-*-gnu to be configured with 64-bit IEEE-compliant `long double` (the same as ordinary `double`) rather than the nonstandard, non-IEEE "IBM double" format from AIX. Support for using only IEEE 128-bit doubles in glibc is not yet ready. This resolves several package test failures caused by weird nonstandard floating-point semantics. It also eliminates a lot of difficulties that arise when using glibc-linked tools to compile for musl-libc and vice versa. There is a long comment included in gcc/common/platform-flags.nix explaining the situation and the reasoning.

vcunat · 2022-04-29T06:46:50Z

pkgs/development/compilers/gcc/10/default.nix

@@ -67,6 +67,7 @@ let majorVersion = "10";
      ++ optional langAda ../gnat-cflags.patch
      ++ optional langFortran ../gfortran-driving.patch
      ++ optional (targetPlatform.libc == "musl" && targetPlatform.isPower) ../ppc-musl.patch
+      ++ optional (targetPlatform.isMusl         && targetPlatform.isPower64) ./backport-libgcc-fix-PR97653.patch


Perhaps it would help to instead default the platform to gcc11? (like x86_64-linux and some others)

I'll try that as soon as my current rebuild-the-world finishes.

However even in that case could we still keep the line above? At least that way if somebody needs gcc10 for some reason it will work.

PS, that line is actually from #170400, which this PR builds on top of. That PR is ready for review and possible merge; this one is still WIP but getting very close.

ghost · 2022-04-29T08:37:46Z

This is looking really promising.

I have verified that with this PR, the tests for goffice pass. Meaning that if this merges, #170567 can be reverted.

I am optimistic that this is going to save us having to scatter a ton of doCheck = !stdenv.hostPlatform.isPower64's all over the codebase wherever a package's test involves floating point rounding behavior.

ghost · 2022-04-30T09:52:30Z

Everything rebuilt, and I'm running it now.

The only catch is that glibc doesn't want to let itself be built by a compiler that was built using either --without-long-double-128 or --with-long-double-64. So I was able to build everything because stdenv uses the bootstrap-tools's compiler (rather than its own compiler) to build glibc. But I can't rebuild the bootstrap-files.

Other than that, it works.

It may take me a while to resolve the glibc issue.

Also, I noticed that stdenv builds a somewhat Frankensteined compiler... gcc is compiled by itself (using the stage1/stage2 thing that it does as part of its internal bootstrap), but the final (stage2) gcc gets linked against a libgmp, libmpc, libisl, and libmpfr that were compiled by the bootstrap-tools' compiler.

Has there been any effort put into splitting gcc's two-step bootstrap into two separate derivations? Its build machinery has support for that (--disable-bootstrap). Then we could build gmp/mpc/isl/mpfr using gcc-stage1 and provide those to gcc-stage2 to get a uniformly self-compiled compiler. This would probably be an additional "stage" in pkgs/stdenv/linux/default.nix, but the only additional compilation would be one extra recompile of gmp/mpc/isl/mpfr (all of them very quick). In particular, there shouldn't be any extra recompilations of gcc.

vcunat · 2022-04-30T10:34:43Z

I'm not aware of such efforts. I wouldn't expect issues from some parts being built by a bit older compiler. There are many cycles that are hard to avoid among stdenv parts, e.g. also glibc.

ghost · 2022-05-01T00:01:28Z

I'm not aware of such efforts.

I might look into writing a PR for it.

I wouldn't expect issues from some parts being built by a bit older compiler.

It's only a problem when there is an ABI change without a new multiarch tuple. In theory that should never happen. But this whole IBM long double mess appears to be a possible counterexample.

ghost · 2022-05-11T08:23:01Z

So Fedora finally managed to ship a release with IEEE long doubles after attempting six times: this bug is marked CLOSED CURRENTRELEASE.

The word on the street is that this stuff simply did not work until glibc 2.35. That would definitely explain the bruise on my head and the damage to my office wall as a result of one being banged against the other.

I will revisit this after nixpkgs' glibc upgrades to glibc 2.35 or later.

ghost · 2022-06-29T22:29:39Z

Also, I noticed that stdenv builds a somewhat Frankensteined compiler... gcc is compiled by itself (using the stage1/stage2 thing that it does as part of its internal bootstrap), but the final (stage2) gcc gets linked against a libgmp, libmpc, libisl, and libmpfr that were compiled by the bootstrap-tools' compiler.

This didn't sit well with me, but until now I couldn't articulate why.

wouldn't expect issues from some parts being built by a bit older compiler.

I think I found two examples:

The current situation prevents you from using the bootstrap-tools from the 32-bit version of an architecture to bootstrap a 64-bit stdenv on a machine that can execute both 32-bit and 64-bit code. The bootstrap process will attempt to link 32-bit lib{gmp,mpc,isl,mpfr} into the final 64-bit gcc.

Some people will undoubtedly say "oh, 32-bit is so unmodern, nobody cares about that ancient stuff". But:

The same situation comes up with ABIs: both x86 and MIPS have a Knuthian "64-bit integers with 32-bit pointers" ABI. Neither is an earlier/later "version of" the other (like with the powerpc long-double stuff), and neither is strictly-superior/inferior-to the other (so you can't assume one will "kill off" the other at some point).

In both the 32/64-bit case and the ABI case you can always bootstrap a complete nixpkgs using the bootstrap-files you have (e.g. bootstrap a complete 32-bit nixpkgs on a 64-bit processor) and then use that to "cross-compile" your own (e.g. 64-bit) bootstrap-files. So it isn't a major problem. But it is a reason to prefer having the stage2 gcc be compiled entirely by the stage1 gcc, if it were easy to do that. Right now it probably isn't easy.

Edit, 2022-Jul-20: @trofi found another one: #181943 (comment)

ghost · 2022-07-24T21:58:04Z

Closing in favor of #170215 which is now feasible due to glibc-2.35.

Adam Joseph and others added 7 commits April 19, 2022 17:09

Update pkgs/stdenv/linux/default.nix

502d902

Co-authored-by: sternenseemann <sternenseemann@systemli.org>

stdenv: add -stageX markers to gmp, mpfr, libmpc, and isl

5d2aa78

github-actions bot added the 6.topic: stdenv Standard environment label Apr 29, 2022

vcunat reviewed Apr 29, 2022

View reviewed changes

ofborg bot added the 10.rebuild-linux-stdenv This PR causes stdenv to rebuild label Apr 29, 2022

ofborg bot requested review from dasJ, Ericson2314, edolstra, Synthetica9 and vcunat April 29, 2022 08:46

ofborg bot added 10.rebuild-darwin: 0 This PR does not cause any packages to rebuild on Darwin 10.rebuild-linux: 501+ 10.rebuild-linux: 5001+ labels Apr 29, 2022

ghost mentioned this pull request Jul 21, 2022

gcc: pass --with-build-sysroot=/ #181943

Merged

13 tasks

ghost closed this Jul 24, 2022

This was referenced Jan 7, 2023

Bootstrap Reform and aarch64-linux's GCC Upgrade #208412

Closed

stdenv: Nix-driven bootstrap of gcc #209870

Merged

ghost deleted the powerpc64-use-long-double+ branch January 23, 2024 06:47

Artturin mentioned this pull request Jan 29, 2024

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gcc: if isPower64, add --with-long-double-64 --without-long-double-128 #170857

gcc: if isPower64, add --with-long-double-64 --without-long-double-128 #170857

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gcc: if isPower64, add --with-long-double-64 --without-long-double-128 #170857

gcc: if isPower64, add --with-long-double-64 --without-long-double-128 #170857

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ghost commented Apr 29, 2022 • edited by ghost Loading

Description of changes

Description of changes

Background

Notes to be integrated into final commit message

Things done

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