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Remove legacy bitcode for iOS #133297

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merged 2 commits into from
Nov 25, 2024
Merged

Remove legacy bitcode for iOS #133297

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3a23669
embed-bitcode is no longer used in iOS
dianqk Nov 20, 2024
7cc5fee
Remove forces_embed_bitcode
dianqk Nov 20, 2024
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19 changes: 1 addition & 18 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 @@ -955,24 +955,7 @@ pub(crate) fn bitcode_section_name(cgcx: &CodegenContext<LlvmCodegenBackend>) ->
}
}

/// 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
/// code at least with `-fembed-bitcode` which creates two sections in the
/// executable:
///
/// * __LLVM,__bitcode
/// * __LLVM,__cmdline
///
/// It appears *both* of these sections are necessary to get the linker to
/// recognize what's going on. A suitable cmdline value is taken from the
/// target spec.
///
/// Furthermore debug/O1 builds don't actually embed bitcode but rather just
/// embed an empty section.
///
/// Basically all of this is us attempting to follow in the footsteps of clang
/// on iOS. See #35968 for lots more info.
/// Embed the bitcode of an LLVM module for LTO in the LLVM module itself.
unsafe fn embed_bitcode(
cgcx: &CodegenContext<LlvmCodegenBackend>,
llcx: &llvm::Context,
Expand Down
6 changes: 2 additions & 4 deletions compiler/rustc_codegen_ssa/src/back/write.rs
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Expand Up @@ -432,11 +432,9 @@ struct CompiledModules {

fn need_bitcode_in_object(tcx: TyCtxt<'_>) -> bool {
let sess = tcx.sess;
let requested_for_rlib = sess.opts.cg.embed_bitcode
sess.opts.cg.embed_bitcode
&& tcx.crate_types().contains(&CrateType::Rlib)
&& sess.opts.output_types.contains_key(&OutputType::Exe);
let forced_by_target = sess.target.forces_embed_bitcode;
requested_for_rlib || forced_by_target
&& sess.opts.output_types.contains_key(&OutputType::Exe)
}

fn need_pre_lto_bitcode_for_incr_comp(sess: &Session) -> bool {
Expand Down
5 changes: 0 additions & 5 deletions compiler/rustc_target/src/spec/mod.rs
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Expand Up @@ -2327,8 +2327,6 @@ pub struct TargetOptions {
/// If we give emcc .o files that are actually .bc files it
/// will 'just work'.
pub obj_is_bitcode: bool,
/// Whether the target requires that emitted object code includes bitcode.
pub forces_embed_bitcode: bool,
/// Content of the LLVM cmdline section associated with embedded bitcode.
pub bitcode_llvm_cmdline: StaticCow<str>,

Expand Down Expand Up @@ -2671,7 +2669,6 @@ impl Default for TargetOptions {
allow_asm: true,
has_thread_local: false,
obj_is_bitcode: false,
forces_embed_bitcode: false,
bitcode_llvm_cmdline: "".into(),
min_atomic_width: None,
max_atomic_width: None,
Expand Down Expand Up @@ -3412,7 +3409,6 @@ impl Target {
key!(main_needs_argc_argv, bool);
key!(has_thread_local, bool);
key!(obj_is_bitcode, bool);
key!(forces_embed_bitcode, bool);
key!(bitcode_llvm_cmdline);
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LLVM still leave some code related to llvmcmd, but I believe this will also be removed in the future.

key!(max_atomic_width, Option<u64>);
key!(min_atomic_width, Option<u64>);
Expand Down Expand Up @@ -3687,7 +3683,6 @@ impl ToJson for Target {
target_option_val!(main_needs_argc_argv);
target_option_val!(has_thread_local);
target_option_val!(obj_is_bitcode);
target_option_val!(forces_embed_bitcode);
target_option_val!(bitcode_llvm_cmdline);
target_option_val!(min_atomic_width);
target_option_val!(max_atomic_width);
Expand Down
13 changes: 5 additions & 8 deletions src/doc/rustc/src/codegen-options/index.md
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Expand Up @@ -119,17 +119,14 @@ files. It takes one of the following values:
* `n`, `no`, `off` or `false`: omit bitcode from rlibs.

LLVM bitcode is required when rustc is performing link-time optimization (LTO).
It is also required on some targets like iOS ones where vendors look for LLVM
bitcode. Embedded bitcode will appear in rustc-generated object files inside of
a section whose name is defined by the target platform. Most of the time this is
`.llvmbc`.
Embedded bitcode will appear in rustc-generated object files inside of a section
whose name is defined by the target platform. Most of the time this is `.llvmbc`.

The use of `-C embed-bitcode=no` can significantly improve compile times and
reduce generated file sizes if your compilation does not actually need bitcode
(e.g. if you're not compiling for iOS or you're not performing LTO). For these
reasons, Cargo uses `-C embed-bitcode=no` whenever possible. Likewise, if you
are building directly with `rustc` we recommend using `-C embed-bitcode=no`
whenever you are not using LTO.
(e.g. if you're not performing LTO). For these reasons, Cargo uses `-C embed-bitcode=no`
whenever possible. Likewise, if you are building directly with `rustc` we recommend
using `-C embed-bitcode=no` whenever you are not using LTO.

If combined with `-C lto`, `-C embed-bitcode=no` will cause `rustc` to abort
at start-up, because the combination is invalid.
Expand Down
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