Add unstable hotpatch flag to rustc

[nebulark]

This PR adds an unstable hotpatch flag, which ensures the first instruction of a function being two bytes long, otherwise inserting a nop. This is needed for code hotpatching (such as Live++ or Recode) tools to work, so they can provide very quick iteration cycles for development.

This uses a LLVM feature that is only implemented on x86/x86_64. On aarch64 this is not needed as there are no 1 byte instructions.

Hotpatching also requires a linker argument (e.g. functionpadmin on link.exe/lld), which ensures a minimum padding between functions. This can not implemented in rustc as this is linker specific. However, we do mark functions to be hotpatchable when using the hotpatch flag, as the link.exe/lld need it for their functionpadmin flag to work.

Typical usage with cargo for hotpatching the would be "RUSTFLAGS=-Z hotpatch -C link-arg=-functionpadmin". This currently only works on Windows due to linker support. But there should be nothing blocking other linkers to implement a flag similar to functionpadmin.

There is a similar flag "patchable-function-entry". The main difference is that hotpatch + functionpadmin, should leave functions that are already hotpatchable, an extremely common case, untouched. This reduces bloat, but requires linker support.

This is a first step to establish a proof of concept for rust-lang/compiler-team#745. The follow up step would be to add support for lld (or another linker) so this also works on linux.

[rustbot]

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[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

The run-make-support library was changed

cc @jieyouxu

This PR modifies tests/run-make/. If this PR is trying to port a Makefile
run-make test to use rmake.rs, please update the
run-make port tracking issue
so we can track our progress. You can either modify the tracking issue
directly, or you can comment on the tracking issue and link this PR.

cc @jieyouxu

[jieyouxu]

This is waiting on the MCP rust-lang/compiler-team#745, right?

[nebulark]

The MCP is stuck due to missing linux support, due to missing linker support.

Current plan to get the MCP approved would be to:

1. Add this as an unstable flag, which as far as I was told does need need an MCP. So I am just doing it with this PR.
2. Add support to some Linux/ELF linker (probably LLD as the Window/COFF version already supports it)

With this there would be a proof of concept that works on linux and would unstuck the MCP.
Sorry if this caused any confusion.

[jieyouxu]

Ah okay, thanks for the clarifications!

[nebulark]

So ... I think we should remove the "S-waiting-on-MCP" tag then and put it back to waiting on review?
Or is there action needed from my side?

[jieyouxu]

This is waiting on the MCP because the MCP needs a second from a compiler team member. There's no action needed on your side.

[nebulark]

As far as I understood getting a second will only happen when I have a poc that works on linux.
I have been told adding an unstable flag should not require an MCP. Only stabilizing that flag does.
So I am trying to add the unstable flag now.
Please correct me if I understood anything wrong.

Again sorry for the confusion and thank you for your time!

[jieyouxu]

Let me go ask other compiler people about this and then get back to you.

[nikic]

As far as I understood getting a second will only happen when I have a poc that works on linux. I have been told adding an unstable flag should not require an MCP. Only stabilizing that flag does. So I am trying to add the unstable flag now. Please correct me if I understood anything wrong.

I think there's been a misunderstanding here. That was my opinion on the MCP, not on whether it needs an MCP. Unstable flags usually do go through an MCP.

[bors]

☔ The latest upstream changes (presumably #142689) made this pull request unmergeable. Please resolve the merge conflicts.

[estebank]

Shouldn't this warn/error if both conditions aren't true?

[rust-log-analyzer]

The job aarch64-gnu-llvm-19-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

test [run-make] tests/run-make/foreign-exceptions ... ok
test [run-make] tests/run-make/foreign-rust-exceptions ... ok
test [run-make] tests/run-make/glibc-symbols-x86_64-unknown-linux-gnu ... ignored, only executed when the target is x86_64-unknown-linux-gnu
test [run-make] tests/run-make/glibc-staticlib-args ... ok
test [run-make] tests/run-make/hotpatch ... FAILED
test [run-make] tests/run-make/hotpatch_pdb ... ok
test [run-make] tests/run-make/ice-dep-cannot-find-dep ... ignored, only executed when the architecture is x86_64
test [run-make] tests/run-make/hotpatch-unaffected ... ok
test [run-make] tests/run-make/import-macro-verbatim ... ignored, only executed when the operating system is windows (other platforms do not have Windows verbatim paths)
test [run-make] tests/run-make/ice-static-mir ... ok
test [run-make] tests/run-make/inaccessible-temp-dir ... ok
test [run-make] tests/run-make/include-bytes-deps ... ok
test [run-make] tests/run-make/include-all-symbols-linking ... ok
---
---- [run-make] tests/run-make/hotpatch stdout ----

error: rmake recipe failed to complete
status: exit status: 1
command: cd "/checkout/obj/build/aarch64-unknown-linux-gnu/test/run-make/hotpatch/rmake_out" && env -u RUSTFLAGS AR="ar" BUILD_ROOT="/checkout/obj/build/aarch64-unknown-linux-gnu" CARGO="/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools-bin/cargo" CC="cc" CC_DEFAULT_FLAGS="-ffunction-sections -fdata-sections -fPIC" CXX="c++" CXX_DEFAULT_FLAGS="-ffunction-sections -fdata-sections -fPIC" HOST_RUSTC_DYLIB_PATH="/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/lib" LD_LIBRARY_PATH="/checkout/obj/build/aarch64-unknown-linux-gnu/bootstrap-tools/aarch64-unknown-linux-gnu/release/deps:/checkout/obj/build/aarch64-unknown-linux-gnu/stage0/lib/rustlib/aarch64-unknown-linux-gnu/lib" LD_LIB_PATH_ENVVAR="LD_LIBRARY_PATH" LLVM_BIN_DIR="/usr/lib/llvm-19/bin" LLVM_COMPONENTS="aarch64 aarch64asmparser aarch64codegen aarch64desc aarch64disassembler aarch64info aarch64utils aggressiveinstcombine all all-targets amdgpu amdgpuasmparser amdgpucodegen amdgpudesc amdgpudisassembler amdgpuinfo amdgputargetmca amdgpuutils analysis arm armasmparser armcodegen armdesc armdisassembler arminfo armutils asmparser asmprinter avr avrasmparser avrcodegen avrdesc avrdisassembler avrinfo binaryformat bitreader bitstreamreader bitwriter bpf bpfasmparser bpfcodegen bpfdesc bpfdisassembler bpfinfo cfguard codegen codegendata codegentypes core coroutines coverage debuginfobtf debuginfocodeview debuginfodwarf debuginfogsym debuginfologicalview debuginfomsf debuginfopdb demangle dlltooldriver dwarflinker dwarflinkerclassic dwarflinkerparallel dwp engine executionengine extensions filecheck frontenddriver frontendhlsl frontendoffloading frontendopenacc frontendopenmp fuzzercli fuzzmutate globalisel hexagon hexagonasmparser hexagoncodegen hexagondesc hexagondisassembler hexagoninfo hipstdpar instcombine instrumentation interfacestub interpreter ipo irprinter irreader jitlink lanai lanaiasmparser lanaicodegen lanaidesc lanaidisassembler lanaiinfo libdriver lineeditor linker loongarch loongarchasmparser loongarchcodegen loongarchdesc loongarchdisassembler loongarchinfo lto m68k m68kasmparser m68kcodegen m68kdesc m68kdisassembler m68kinfo mc mca mcdisassembler mcjit mcparser mips mipsasmparser mipscodegen mipsdesc mipsdisassembler mipsinfo mirparser msp430 msp430asmparser msp430codegen msp430desc msp430disassembler msp430info native nativecodegen nvptx nvptxcodegen nvptxdesc nvptxinfo objcarcopts objcopy object objectyaml option orcdebugging orcjit orcshared orctargetprocess passes perfjitevents powerpc powerpcasmparser powerpccodegen powerpcdesc powerpcdisassembler powerpcinfo profiledata remarks riscv riscvasmparser riscvcodegen riscvdesc riscvdisassembler riscvinfo riscvtargetmca runtimedyld sandboxir scalaropts selectiondag sparc sparcasmparser sparccodegen sparcdesc sparcdisassembler sparcinfo support symbolize systemz systemzasmparser systemzcodegen systemzdesc systemzdisassembler systemzinfo tablegen target targetparser textapi textapibinaryreader transformutils ve veasmparser vecodegen vectorize vedesc vedisassembler veinfo webassembly webassemblyasmparser webassemblycodegen webassemblydesc webassemblydisassembler webassemblyinfo webassemblyutils windowsdriver windowsmanifest x86 x86asmparser x86codegen x86desc x86disassembler x86info x86targetmca xcore xcorecodegen xcoredesc xcoredisassembler xcoreinfo xray xtensa xtensaasmparser xtensacodegen xtensadesc xtensadisassembler xtensainfo" LLVM_FILECHECK="/usr/lib/llvm-19/bin/FileCheck" NODE="/usr/bin/node" PYTHON="/usr/bin/python3" RUSTC="/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" RUSTDOC="/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustdoc" SOURCE_ROOT="/checkout" TARGET="aarch64-unknown-linux-gnu" TARGET_EXE_DYLIB_PATH="/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/lib/rustlib/aarch64-unknown-linux-gnu/lib" __RUSTC_DEBUG_ASSERTIONS_ENABLED="1" __STD_DEBUG_ASSERTIONS_ENABLED="1" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/run-make/hotpatch/rmake"
stdout: none
--- stderr -------------------------------
command failed at line 39
Command { cmd: "/usr/lib/llvm-19/bin/FileCheck" "lib.rs" "--check-prefix" "HOTPATCH", stdin_buf: Some([10, 108, 105, 98, 104, 111, 116, 112, 97, 116, 99, 104, 46, 114, 108, 105, 98, 40, 108, 105, 98, 46, 114, 109, 101, 116, 97, 41, 58, 9, 102, 105, 108, 101, 32, 102, 111, 114, 109, 97, 116, 32, 101, 108, 102, 54, 52, 45, 108, 105, 116, 116, 108, 101, 97, 97, 114, 99, 104, 54, 52, 10, 10, 108, 105, 98, 104, 111, 116, 112, 97, 116, 99, 104, 46, 114, 108, 105, 98, 40, 104, 111, 116, 112, 97, 116, 99, 104, 46, 104, 111, 116, 112, 97, 116, 99, 104, 46, 54, 97, 57, 48, 101, 102, 98, 49, 54, 97, 50, 101, 100, 97, 54, 52, 45, 99, 103, 117, 46, 48, 46, 114, 99, 103, 117, 46, 111, 41, 58, 9, 102, 105, 108, 101, 32, 102, 111, 114, 109, 97, 116, 32, 101, 108, 102, 54, 52, 45, 108, 105, 116, 116, 108, 101, 97, 97, 114, 99, 104, 54, 52, 10, 10, 68, 105, 115, 97, 115, 115, 101, 109, 98, 108, 121, 32, 111, 102, 32, 115, 101, 99, 116, 105, 111, 110, 32, 46, 116, 101, 120, 116, 46, 101, 109, 112, 116, 121, 95, 102, 110, 58, 10, 10, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48, 48, 32, 60, 101, 109, 112, 116, 121, 95, 102, 110, 62, 58, 10, 32, 32, 32, 32, 32, 32, 32, 48, 58, 32, 100, 54, 53, 102, 48, 51, 99, 48, 32, 32, 32, 32, 32, 9, 114, 101, 116, 10]), stdin: None, stdout: None, stderr: None, drop_bomb: DropBomb { command: "/usr/lib/llvm-19/bin/FileCheck", defused: true, armed_location: Location { file: "/checkout/tests/run-make/hotpatch/rmake.rs", line: 35, column: 13 } }, already_executed: true }
output status: `exit status: 1`
=== STDOUT ===



=== STDERR ===
lib.rs:23:19: error: HOTPATCH-NEXT: expected string not found in input
// HOTPATCH-NEXT: 0: {{[0-9a-f][0-9a-f]}} {{[0-9a-f][0-9a-f]}} {{.*}}
                  ^
<stdin>:8:29: note: scanning from here
0000000000000000 <empty_fn>:
                            ^
<stdin>:9:2: note: possible intended match here
 0: d65f03c0 ret
 ^

Input file: <stdin>
Check file: lib.rs

-dump-input=help explains the following input dump.

Input was:
<<<<<<
           1:  
           2: libhotpatch.rlib(lib.rmeta): file format elf64-littleaarch64 
           3:  
           4: libhotpatch.rlib(hotpatch.hotpatch.6a90efb16a2eda64-cgu.0.rcgu.o): file format elf64-littleaarch64 
           5:  
           6: Disassembly of section .text.empty_fn: 
           7:  
           8: 0000000000000000 <empty_fn>: 
next:23'0                                 X error: no match found
           9:  0: d65f03c0 ret 
next:23'0     ~~~~~~~~~~~~~~~~~
next:23'1      ?                possible intended match
>>>>>>
------------------------------------------