Add support for generating uninitialized sections #306

MichalStrehovsky · 2022-11-04T09:03:53Z

We support .bss but not custom sections that are bss-like. This adds such support.

We support `.bss` but not custom sections that are bss-like. This adds such support.

filipnavara

LGTM

VSadov · 2022-11-07T03:26:54Z

llvm/tools/objwriter/objwriter.cpp

@@ -260,6 +265,9 @@ MCSection *ObjectWriter::GetSpecificSection(const char *SectionName,
      // boundaries.
      typeAndAttributes |= MachO::S_ATTR_PURE_INSTRUCTIONS;
    }
+    if (attributes & CustomSectionAttributes_Uninitialized) {
+      typeAndAttributes |= MachO::S_ZEROFILL;


Is zerofill needed for correctness or just MachO wants that for uninitialized data?

Without that we were generating a lot of zeros in the object file I generated.

It does the same thing as COFF::IMAGE_SCN_CNT_UNINITIALIZED_DATA for PE and ELF::SHT_NOBITS for ELF below. But yeah, I don't know why LLVM has both these format specific enums and SectionKind above.

I haven't actually tried end-to-end on mac, but the object file looks about right now in llvm-objdump.

I see. I am not familiar with MachO at this level. I guess it needs to fill the space with zeroes one way or another. So it is either zeroes in the obj file or a fill it with zeroes at load/demand time. The flag could be selecting that.
Makes sense.

VSadov · 2022-11-07T05:16:15Z

llvm/tools/objwriter/objwriter.cpp

@@ -299,8 +310,11 @@ MCSection *ObjectWriter::GetSpecificSection(const char *SectionName,
    } else if (attributes & CustomSectionAttributes_Writeable) {
      Flags |= ELF::SHF_WRITE;
    }
+    unsigned SectionType = (attributes & CustomSectionAttributes_Uninitialized)
+                               ? ELF::SHT_NOBITS


for BSS section the SHT_NOBITS should be combined with SHF_ALLOC, to be present in memory, which we get from line 301 above and SHF_WRITE, which we get because we set the writeable attribute in object writer at:

case SectionType.Uninitialized: attributes |= CustomSectionAttributes.Uninitialized | CustomSectionAttributes.Writeable; break;

Looks right to me.

We support `.bss` but not custom sections that are bss-like. This adds such support.

@EgorBo

* Apply llvm.patch Taken from https://github.com/dotnet/runtime/blob/7ab969c84ef05ba948c0075392716ce335b47744/src/coreclr/tools/aot/ObjWriter/llvm.patch. * Add objwriter library * Taken from https://github.com/dotnet/runtime/tree/7ab969c84ef05ba948c0075392716ce335b47744/src/coreclr/tools/aot/ObjWriter. * Updated README.md * Updated CMakeLists.txt to remove reference to CORECLR_INCLUDE_DIR. * Added cordebuginfo.h, cvconst.h, cfi.h from coreclr/inc at the above commit. * Build the ObjWriter package * Add ObjWriter API to set DWARF version (#161) Contributes to https://github.com/dotnet/runtimelab/issues/1738. * Add `.note.GNU-stack` section to produced executables (#162) Do this unconditionally because there's no scenario where we would need executable stack for managed code. * Remove Darwin workaround (#163) This caught my attention as I was looking at the ObjWriter. LLVM no longer emits a `LC_VERSION_MIN_MACOSX` load command unless we explicitly set a version. I don't see a difference in `llvm-objdump -macho -x foo.o` with/without these lines (I didn't bother myself to boot into macOS to run `otool`). * Fix llvm-dwarfdump warnings (#164) Fixes https://github.com/dotnet/runtimelab/issues/1535. No warnings left with llvm-dwarfdump from LLVM 12. * Revert "Fix llvm-dwarfdump warnings (#164)" (#218) This reverts commit afc9070. * Add new NuGet package, `Microsoft.NETCore.Runtime.JIT.Tools`, includes `FileCheck` and `llvm-mca` (#256) https://github.com/dotnet/runtime is wanting to start writing assembly (x64/ARM64) verification tests. Instead of building our own tool to support writing those kinds of tests, we want to leverage LLVM's `FileCheck`. We also want to include `llvm-mca` at the request of @EgorBo This PR creates a new NuGet package for `dotnet/runtime` to consume which we named `Microsoft.NETCore.Runtime.JIT.Tools`. So far, this package only includes LLVM's `FileCheck` and `llvm-mca` tools. * [ObjWriter] Enable DWARF debug information emitting for Mach-O (#269) * Account for GOT VariantKind on osx-arm64 (#185) * Add API for emitting compact unwind encoding, enforce DWARF encoding if not explicitly overridden * Add comment * Update ObjWriter to LLVM 14 API * Add support for generating uninitialized sections (#306) We support `.bss` but not custom sections that are bss-like. This adds such support. * Do not indiscriminately create text section (#312) If we ended up with nothing in the text section, this line would error LLVM out in: https://github.com/dotnet/llvm-project/blob/3db8d68195c17386557f1a258312bbae4051dc05/llvm/lib/MC/ELFObjectWriter.cpp#L1458-L1459 Because we generate a reference to the empty text section in the `aranges` section. I double checked and debugging on Linux still works fine without this. `SetCodeSectionAttribute` is an objwriter API and we have access to it from the managed side. We should be calling it from there if it's needed for something that I didn't realize (we do call it from the managed side for the `.managed` section, but that one actually has debug information generated, unlike `.text`). * Fix off-by-one error in DWARF reg-reg location (#317) The DWARF specification states that the form of an exprloc consists of an unsigned LEB128 length value, followed by the encoded location bytes of the specified length. For some reason we were adding one to the length value being emitted. This looks incorrect to me. The above calculation for REG-REG (a variable stored in two registers) correctly calculates the length of each register type tag, plus the size of the interpolating PIECE tags, plus the size of notation for each register. The extra byte looks wrong. I've tested this locally and it appears to resolve dotnet/runtime#77407. Unfortunately, it also causes llvm-dwarfdump --verify to constantly complain about missing base addresses. I can't confirm at the moment, but my suspicion is that this is revealing an existing bug. Even if this is somehow causing a new bug, I think the resulting symbols with this change are better than the alternative (no working symbols at all). * Setting context object file info * Add verbosity to linux x64 pipeline In order to understand what is happening with std path error. * Revert "Add verbosity to linux x64 pipeline" This reverts commit 5c4636e. * Upgrading linux build image * [Temporary] Adding verbosity to get more pipeline error info * Update image name for linux x64 * Fix Linux x64 build * Revert "[Temporary] Adding verbosity to get more pipeline error info" This reverts commit 9d76b36. * Updating Build_Linux_musl timeout * Update linux-musl Docker images * Fix linux-musl-x64 build * Setting clang/++ version 15 for linux musl * Copying clang/clan++ vars to unix-like OS * Fix cut & paste error * Fix objcopy and strip path in cross-compilation * Update azure-pipelines.yml $(ClangVersion) $(ClangPlusVersion) weren't defined for OSX and should be defined for every Linux * Bump timeout for Linux musl build * Clean up .gitignore * Consolidate Clang[Plus]Version into ClangVersionArg * Move CLANG_TARGET from environment into build parameter Always quote _BuildConfig on command line so empty value is not accidentally using next parameter as the value * Update URL in cordebuginfo.h to point to dotnet/runtime * Bump Windows build timeout to 210 * Fix a typo in compiler name * Revert $(_BuildConfig) -> "$(_BuildConfig)" change * Change ClangTarget to ClangTargetArg since apparently it gets propagated as environment variable into wrong steps * Fix inadvertent change * Bump timeout everywhere --------- Co-authored-by: Michal Strehovský <MichalStrehovsky@users.noreply.github.com> Co-authored-by: Andy Gocke <andy@commentout.net> Co-authored-by: Will Smith <lol.tihan@gmail.com> Co-authored-by: Adeel Mujahid <3840695+am11@users.noreply.github.com> Co-authored-by: Brian Bohe <brianbohe@gmail.com> Co-authored-by: Alexander Köplinger <alex.koeplinger@outlook.com>

Add support for generating uninitialized sections

f3c28e9

We support `.bss` but not custom sections that are bss-like. This adds such support.

MichalStrehovsky mentioned this pull request Nov 4, 2022

Add support for dehydrated runtime data structures dotnet/runtime#77884

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filipnavara approved these changes Nov 4, 2022

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VSadov reviewed Nov 7, 2022

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VSadov approved these changes Nov 7, 2022

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MichalStrehovsky merged commit 3db8d68 into dotnet:objwriter/12.x Nov 8, 2022

MichalStrehovsky deleted the uninitSection branch November 8, 2022 06:32

filipnavara pushed a commit to filipnavara/llvm-project that referenced this pull request Nov 16, 2022

Add support for generating uninitialized sections (dotnet#306)

0643adf

We support `.bss` but not custom sections that are bss-like. This adds such support.

MichalStrehovsky added a commit to bflattened/llvm-project that referenced this pull request Nov 17, 2022

Add support for generating uninitialized sections (dotnet#306)

4616a47

We support `.bss` but not custom sections that are bss-like. This adds such support.

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Add support for generating uninitialized sections #306

Add support for generating uninitialized sections #306

MichalStrehovsky commented Nov 4, 2022

filipnavara left a comment

VSadov Nov 7, 2022

MichalStrehovsky Nov 7, 2022 •

edited

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VSadov Nov 7, 2022

VSadov Nov 7, 2022

Add support for generating uninitialized sections #306

Add support for generating uninitialized sections #306

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MichalStrehovsky commented Nov 4, 2022

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VSadov Nov 7, 2022

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MichalStrehovsky Nov 7, 2022 • edited Loading

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VSadov Nov 7, 2022

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MichalStrehovsky Nov 7, 2022 •

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