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Use module inline assembly to embed bitcode #91654

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Dec 13, 2021
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Use module inline assembly to embed bitcode #91654

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102 changes: 60 additions & 42 deletions compiler/rustc_codegen_llvm/src/back/write.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -954,6 +954,29 @@ pub(crate) unsafe fn codegen(
))
}

fn create_section_with_flags_asm(section_name: &str, section_flags: &str, data: &[u8]) -> Vec<u8> {
let mut asm = format!(".section {},\"{}\"\n", section_name, section_flags).into_bytes();
asm.extend_from_slice(b".ascii \"");
asm.reserve(data.len());
for &byte in data {
if byte == b'\\' || byte == b'"' {
asm.push(b'\\');
asm.push(byte);
} else if byte < 0x20 || byte >= 0x80 {
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To be honest, I'm somewhat more comfortable with a .byte directive over .ascii with escapes here. Is there any particular reason why we'd be stuck with .ascii?

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.byte should work as well. I picked .ascii here for more compact encoding. Though I guess .byte isn't so bad when using decimal rather than hex and saving on the 0x prefix for each byte.

// Avoid non UTF-8 inline assembly. Use octal escape sequence, because it is fixed
// width, while hex escapes will consume following characters.
asm.push(b'\\');
asm.push(b'0' + ((byte >> 6) & 0x7));
asm.push(b'0' + ((byte >> 3) & 0x7));
asm.push(b'0' + ((byte >> 0) & 0x7));
} else {
asm.push(byte);
}
}
asm.extend_from_slice(b"\"\n");
asm
}

/// Embed the bitcode of an LLVM module in the LLVM module itself.
///
/// This is done primarily for iOS where it appears to be standard to compile C
Expand All @@ -979,34 +1002,6 @@ unsafe fn embed_bitcode(
cmdline: &str,
bitcode: &[u8],
) {
let llconst = common::bytes_in_context(llcx, bitcode);
let llglobal = llvm::LLVMAddGlobal(
llmod,
common::val_ty(llconst),
"rustc.embedded.module\0".as_ptr().cast(),
);
llvm::LLVMSetInitializer(llglobal, llconst);

let is_apple = cgcx.opts.target_triple.triple().contains("-ios")
|| cgcx.opts.target_triple.triple().contains("-darwin")
|| cgcx.opts.target_triple.triple().contains("-tvos");

let section = if is_apple { "__LLVM,__bitcode\0" } else { ".llvmbc\0" };
llvm::LLVMSetSection(llglobal, section.as_ptr().cast());
llvm::LLVMRustSetLinkage(llglobal, llvm::Linkage::PrivateLinkage);
llvm::LLVMSetGlobalConstant(llglobal, llvm::True);

let llconst = common::bytes_in_context(llcx, cmdline.as_bytes());
let llglobal = llvm::LLVMAddGlobal(
llmod,
common::val_ty(llconst),
"rustc.embedded.cmdline\0".as_ptr().cast(),
);
llvm::LLVMSetInitializer(llglobal, llconst);
let section = if is_apple { "__LLVM,__cmdline\0" } else { ".llvmcmd\0" };
llvm::LLVMSetSection(llglobal, section.as_ptr().cast());
llvm::LLVMRustSetLinkage(llglobal, llvm::Linkage::PrivateLinkage);

// We're adding custom sections to the output object file, but we definitely
// do not want these custom sections to make their way into the final linked
// executable. The purpose of these custom sections is for tooling
Expand All @@ -1028,31 +1023,54 @@ unsafe fn embed_bitcode(
// * COFF - if we don't do anything the linker will by default copy all
// these sections to the output artifact, not what we want! To subvert
// this we want to flag the sections we inserted here as
// `IMAGE_SCN_LNK_REMOVE`. Unfortunately though LLVM has no native way to
// do this. Thankfully though we can do this with some inline assembly,
// which is easy enough to add via module-level global inline asm.
// `IMAGE_SCN_LNK_REMOVE`.
//
// * ELF - this is very similar to COFF above. One difference is that these
// sections are removed from the output linked artifact when
// `--gc-sections` is passed, which we pass by default. If that flag isn't
// passed though then these sections will show up in the final output.
// Additionally the flag that we need to set here is `SHF_EXCLUDE`.
//
// Unfortunately, LLVM provides no way to set custom section flags. For ELF
// and COFF we emit the sections using module level inline assembly for that
// reason (see issue #90326 for historical background).
let is_apple = cgcx.opts.target_triple.triple().contains("-ios")
|| cgcx.opts.target_triple.triple().contains("-darwin")
|| cgcx.opts.target_triple.triple().contains("-tvos");
if is_apple
|| cgcx.opts.target_triple.triple().starts_with("wasm")
|| cgcx.opts.target_triple.triple().starts_with("asmjs")
{
// nothing to do here
} else if cgcx.is_pe_coff {
let asm = "
.section .llvmbc,\"n\"
.section .llvmcmd,\"n\"
";
llvm::LLVMRustAppendModuleInlineAsm(llmod, asm.as_ptr().cast(), asm.len());
// We don't need custom section flags, create LLVM globals.
let llconst = common::bytes_in_context(llcx, bitcode);
let llglobal = llvm::LLVMAddGlobal(
llmod,
common::val_ty(llconst),
"rustc.embedded.module\0".as_ptr().cast(),
);
llvm::LLVMSetInitializer(llglobal, llconst);

let section = if is_apple { "__LLVM,__bitcode\0" } else { ".llvmbc\0" };
llvm::LLVMSetSection(llglobal, section.as_ptr().cast());
llvm::LLVMRustSetLinkage(llglobal, llvm::Linkage::PrivateLinkage);
llvm::LLVMSetGlobalConstant(llglobal, llvm::True);

let llconst = common::bytes_in_context(llcx, cmdline.as_bytes());
let llglobal = llvm::LLVMAddGlobal(
llmod,
common::val_ty(llconst),
"rustc.embedded.cmdline\0".as_ptr().cast(),
);
llvm::LLVMSetInitializer(llglobal, llconst);
let section = if is_apple { "__LLVM,__cmdline\0" } else { ".llvmcmd\0" };
llvm::LLVMSetSection(llglobal, section.as_ptr().cast());
llvm::LLVMRustSetLinkage(llglobal, llvm::Linkage::PrivateLinkage);
} else {
let asm = "
.section .llvmbc,\"e\"
.section .llvmcmd,\"e\"
";
// We need custom section flags, so emit module-level inline assembly.
let section_flags = if cgcx.is_pe_coff { "n" } else { "e" };
let asm = create_section_with_flags_asm(".llvmbc", section_flags, bitcode);
llvm::LLVMRustAppendModuleInlineAsm(llmod, asm.as_ptr().cast(), asm.len());
let asm = create_section_with_flags_asm(".llvmcmd", section_flags, cmdline.as_bytes());
llvm::LLVMRustAppendModuleInlineAsm(llmod, asm.as_ptr().cast(), asm.len());
}
}
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