Rollup merge of #71490 - petrochenkov:passcheck2, r=davidtwco

Cleanup and document `-C relocation-model`

As the title says, this is mostly a refactoring and documentation.

One potentially observable change here is that `-C relocation-model=default` now takes the default from the Rust target, rather than from the underlying LLVM target. In other words, `-C relocation-model=default` is now equivalent to not specifying the relocation model on command line at all.
Apparently no one used that option because it has other bugs as well, e.g. PIC `default` wasn't treated as PIC in some places.

Author: Dylan-DPC

src/doc/rustc/src/codegen-options/index.md

@@ -319,11 +319,46 @@ to a valid `.profdata` file. See the chapter on
 
 ## relocation-model
 
-This option lets you choose which
-[relocation](https://en.wikipedia.org/wiki/Relocation_\(computing\)) model to
-use.
+This option controls generation of
+[position-independent code (PIC)](https://en.wikipedia.org/wiki/Position-independent_code).
 
-To find the valid options for this flag, run `rustc --print relocation-models`.
+Supported values for this option are:
+
+#### Primary relocation models
+
+- `static` - non-relocatable code, machine instructions may use absolute addressing modes.
+
+- `pic` - fully relocatable position independent code,
+machine instructions need to use relative addressing modes.  
+Equivalent to the "uppercase" `-fPIC` or `-fPIE` options in other compilers,
+depending on the produced crate types.  
+This is the default model for majority of supported targets.
+
+#### Special relocation models
+
+- `dynamic-no-pic` - relocatable external references, non-relocatable code.  
+Only makes sense on Darwin and is rarely used.  
+If StackOverflow tells you to use this as an opt-out of PIC or PIE, don't believe it,
+use `-C relocation-model=static` instead.
+- `ropi`, `rwpi` and `ropi-rwpi` - relocatable code and read-only data, relocatable read-write data,
+and combination of both, respectively.  
+Only makes sense for certain embedded ARM targets.
+- `default` - relocation model default to the current target.  
+Only makes sense as an override for some other explicitly specified relocation model
+previously set on the command line.
+
+Supported values can also be discovered by running `rustc --print relocation-models`.
+
+#### Linking effects
+
+In addition to codegen effects, `relocation-model` has effects during linking.
+
+If the relocation model is `pic` and the current target supports position-independent executables
+(PIE), the linker will be instructed (`-pie`) to produce one.  
+If the target doesn't support both position-independent and statically linked executables,
+then `-C target-feature=+crt-static` "wins" over `-C relocation-model=pic`,
+and the linker is instructed (`-static`) to produce a statically linked
+but not position-independent executable.
 
 ## remark
 

src/librustc_codegen_llvm/back/write.rs

@@ -7,7 +7,7 @@ use crate::back::profiling::{
 use crate::base;
 use crate::common;
 use crate::consts;
-use crate::context::{get_reloc_model, is_pie_binary};
+use crate::context::all_outputs_are_pic_executables;
 use crate::llvm::{self, DiagnosticInfo, PassManager, SMDiagnostic};
 use crate::llvm_util;
 use crate::type_::Type;
@@ -25,6 +25,7 @@ use rustc_middle::bug;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::config::{self, Lto, OutputType, Passes, Sanitizer, SwitchWithOptPath};
 use rustc_session::Session;
+use rustc_target::spec::RelocModel;
 
 use libc::{c_char, c_int, c_uint, c_void, size_t};
 use std::ffi::CString;
@@ -35,16 +36,6 @@ use std::slice;
 use std::str;
 use std::sync::Arc;
 
-pub const RELOC_MODEL_ARGS: [(&str, llvm::RelocMode); 7] = [
-    ("pic", llvm::RelocMode::PIC),
-    ("static", llvm::RelocMode::Static),
-    ("default", llvm::RelocMode::Default),
-    ("dynamic-no-pic", llvm::RelocMode::DynamicNoPic),
-    ("ropi", llvm::RelocMode::ROPI),
-    ("rwpi", llvm::RelocMode::RWPI),
-    ("ropi-rwpi", llvm::RelocMode::ROPI_RWPI),
-];
-
 pub const CODE_GEN_MODEL_ARGS: &[(&str, llvm::CodeModel)] = &[
     ("small", llvm::CodeModel::Small),
     ("kernel", llvm::CodeModel::Kernel),
@@ -84,19 +75,13 @@ pub fn write_output_file(
     }
 }
 
-pub fn create_informational_target_machine(
-    sess: &Session,
-    find_features: bool,
-) -> &'static mut llvm::TargetMachine {
-    target_machine_factory(sess, config::OptLevel::No, find_features)()
+pub fn create_informational_target_machine(sess: &Session) -> &'static mut llvm::TargetMachine {
+    target_machine_factory(sess, config::OptLevel::No)()
         .unwrap_or_else(|err| llvm_err(sess.diagnostic(), &err).raise())
 }
 
-pub fn create_target_machine(
-    tcx: TyCtxt<'_>,
-    find_features: bool,
-) -> &'static mut llvm::TargetMachine {
-    target_machine_factory(&tcx.sess, tcx.backend_optimization_level(LOCAL_CRATE), find_features)()
+pub fn create_target_machine(tcx: TyCtxt<'_>) -> &'static mut llvm::TargetMachine {
+    target_machine_factory(&tcx.sess, tcx.backend_optimization_level(LOCAL_CRATE))()
         .unwrap_or_else(|err| llvm_err(tcx.sess.diagnostic(), &err).raise())
 }
 
@@ -126,15 +111,22 @@ fn to_pass_builder_opt_level(cfg: config::OptLevel) -> llvm::PassBuilderOptLevel
     }
 }
 
-// If find_features is true this won't access `sess.crate_types` by assuming
-// that `is_pie_binary` is false. When we discover LLVM target features
-// `sess.crate_types` is uninitialized so we cannot access it.
+fn to_llvm_relocation_model(relocation_model: RelocModel) -> llvm::RelocModel {
+    match relocation_model {
+        RelocModel::Static => llvm::RelocModel::Static,
+        RelocModel::Pic => llvm::RelocModel::PIC,
+        RelocModel::DynamicNoPic => llvm::RelocModel::DynamicNoPic,
+        RelocModel::Ropi => llvm::RelocModel::ROPI,
+        RelocModel::Rwpi => llvm::RelocModel::RWPI,
+        RelocModel::RopiRwpi => llvm::RelocModel::ROPI_RWPI,
+    }
+}
+
 pub fn target_machine_factory(
     sess: &Session,
     optlvl: config::OptLevel,
-    find_features: bool,
 ) -> Arc<dyn Fn() -> Result<&'static mut llvm::TargetMachine, String> + Send + Sync> {
-    let reloc_model = get_reloc_model(sess);
+    let reloc_model = to_llvm_relocation_model(sess.relocation_model());
 
     let (opt_level, _) = to_llvm_opt_settings(optlvl);
     let use_softfp = sess.opts.cg.soft_float;
@@ -175,7 +167,7 @@ pub fn target_machine_factory(
     let features = features.join(",");
     let features = CString::new(features).unwrap();
     let abi = SmallCStr::new(&sess.target.target.options.llvm_abiname);
-    let is_pie_binary = !find_features && is_pie_binary(sess);
+    let pic_is_pie = all_outputs_are_pic_executables(sess);
     let trap_unreachable = sess.target.target.options.trap_unreachable;
     let emit_stack_size_section = sess.opts.debugging_opts.emit_stack_sizes;
 
@@ -192,7 +184,7 @@ pub fn target_machine_factory(
                 reloc_model,
                 opt_level,
                 use_softfp,
-                is_pie_binary,
+                pic_is_pie,
                 ffunction_sections,
                 fdata_sections,
                 trap_unreachable,

src/librustc_codegen_llvm/context.rs

@@ -21,7 +21,7 @@ use rustc_session::Session;
 use rustc_span::source_map::{Span, DUMMY_SP};
 use rustc_span::symbol::Symbol;
 use rustc_target::abi::{HasDataLayout, LayoutOf, PointeeInfo, Size, TargetDataLayout, VariantIdx};
-use rustc_target::spec::{HasTargetSpec, Target};
+use rustc_target::spec::{HasTargetSpec, RelocModel, Target};
 
 use std::cell::{Cell, RefCell};
 use std::ffi::CStr;
@@ -87,22 +87,6 @@ pub struct CodegenCx<'ll, 'tcx> {
     local_gen_sym_counter: Cell<usize>,
 }
 
-pub fn get_reloc_model(sess: &Session) -> llvm::RelocMode {
-    let reloc_model_arg = match sess.opts.cg.relocation_model {
-        Some(ref s) => &s[..],
-        None => &sess.target.target.options.relocation_model[..],
-    };
-
-    match crate::back::write::RELOC_MODEL_ARGS.iter().find(|&&arg| arg.0 == reloc_model_arg) {
-        Some(x) => x.1,
-        _ => {
-            sess.err(&format!("{:?} is not a valid relocation mode", reloc_model_arg));
-            sess.abort_if_errors();
-            bug!();
-        }
-    }
-}
-
 fn get_tls_model(sess: &Session) -> llvm::ThreadLocalMode {
     let tls_model_arg = match sess.opts.debugging_opts.tls_model {
         Some(ref s) => &s[..],
@@ -119,12 +103,14 @@ fn get_tls_model(sess: &Session) -> llvm::ThreadLocalMode {
     }
 }
 
-fn is_any_library(sess: &Session) -> bool {
-    sess.crate_types.borrow().iter().any(|ty| *ty != config::CrateType::Executable)
-}
-
-pub fn is_pie_binary(sess: &Session) -> bool {
-    !is_any_library(sess) && get_reloc_model(sess) == llvm::RelocMode::PIC
+/// PIE is potentially more effective than PIC, but can only be used in executables.
+/// If all our outputs are executables, then we can relax PIC to PIE when producing object code.
+/// If the list of crate types is not yet known we conservatively return `false`.
+pub fn all_outputs_are_pic_executables(sess: &Session) -> bool {
+    sess.relocation_model() == RelocModel::Pic
+        && sess.crate_types.try_get().map_or(false, |crate_types| {
+            crate_types.iter().all(|ty| *ty == config::CrateType::Executable)
+        })
 }
 
 fn strip_function_ptr_alignment(data_layout: String) -> String {
@@ -157,7 +143,7 @@ pub unsafe fn create_module(
 
     // Ensure the data-layout values hardcoded remain the defaults.
     if sess.target.target.options.is_builtin {
-        let tm = crate::back::write::create_informational_target_machine(&tcx.sess, false);
+        let tm = crate::back::write::create_informational_target_machine(tcx.sess);
         llvm::LLVMRustSetDataLayoutFromTargetMachine(llmod, tm);
         llvm::LLVMRustDisposeTargetMachine(tm);
 
@@ -200,11 +186,11 @@ pub unsafe fn create_module(
     let llvm_target = SmallCStr::new(&sess.target.target.llvm_target);
     llvm::LLVMRustSetNormalizedTarget(llmod, llvm_target.as_ptr());
 
-    if get_reloc_model(sess) == llvm::RelocMode::PIC {
+    if sess.relocation_model() == RelocModel::Pic {
         llvm::LLVMRustSetModulePICLevel(llmod);
     }
 
-    if is_pie_binary(sess) {
+    if all_outputs_are_pic_executables(sess) {
         llvm::LLVMRustSetModulePIELevel(llmod);
     }
 

src/librustc_codegen_llvm/lib.rs

@@ -110,9 +110,8 @@ impl ExtraBackendMethods for LlvmCodegenBackend {
         &self,
         sess: &Session,
         optlvl: OptLevel,
-        find_features: bool,
     ) -> Arc<dyn Fn() -> Result<&'static mut llvm::TargetMachine, String> + Send + Sync> {
-        back::write::target_machine_factory(sess, optlvl, find_features)
+        back::write::target_machine_factory(sess, optlvl)
     }
     fn target_cpu<'b>(&self, sess: &'b Session) -> &'b str {
         llvm_util::target_cpu(sess)
@@ -201,7 +200,9 @@ impl CodegenBackend for LlvmCodegenBackend {
         match req {
             PrintRequest::RelocationModels => {
                 println!("Available relocation models:");
-                for &(name, _) in back::write::RELOC_MODEL_ARGS.iter() {
+                for name in
+                    &["static", "pic", "dynamic-no-pic", "ropi", "rwpi", "ropi-rwpi", "default"]
+                {
                     println!("    {}", name);
                 }
                 println!();
@@ -351,19 +352,15 @@ impl ModuleLlvm {
         unsafe {
             let llcx = llvm::LLVMRustContextCreate(tcx.sess.fewer_names());
             let llmod_raw = context::create_module(tcx, llcx, mod_name) as *const _;
-            ModuleLlvm { llmod_raw, llcx, tm: create_target_machine(tcx, false) }
+            ModuleLlvm { llmod_raw, llcx, tm: create_target_machine(tcx) }
         }
     }
 
     fn new_metadata(tcx: TyCtxt<'_>, mod_name: &str) -> Self {
         unsafe {
             let llcx = llvm::LLVMRustContextCreate(tcx.sess.fewer_names());
             let llmod_raw = context::create_module(tcx, llcx, mod_name) as *const _;
-            ModuleLlvm {
-                llmod_raw,
-                llcx,
-                tm: create_informational_target_machine(&tcx.sess, false),
-            }
+            ModuleLlvm { llmod_raw, llcx, tm: create_informational_target_machine(tcx.sess) }
         }
     }
 

src/librustc_codegen_llvm/llvm/ffi.rs

@@ -445,8 +445,7 @@ pub struct SanitizerOptions {
 /// LLVMRelocMode
 #[derive(Copy, Clone, PartialEq)]
 #[repr(C)]
-pub enum RelocMode {
-    Default,
+pub enum RelocModel {
     Static,
     PIC,
     DynamicNoPic,
@@ -1946,7 +1945,7 @@ extern "C" {
         Features: *const c_char,
         Abi: *const c_char,
         Model: CodeModel,
-        Reloc: RelocMode,
+        Reloc: RelocModel,
         Level: CodeGenOptLevel,
         UseSoftFP: bool,
         PositionIndependentExecutable: bool,

src/librustc_codegen_llvm/llvm_util.rs

@@ -270,7 +270,7 @@ pub fn to_llvm_feature<'a>(sess: &Session, s: &'a str) -> &'a str {
 }
 
 pub fn target_features(sess: &Session) -> Vec<Symbol> {
-    let target_machine = create_informational_target_machine(sess, true);
+    let target_machine = create_informational_target_machine(sess);
     target_feature_whitelist(sess)
         .iter()
         .filter_map(|&(feature, gate)| {
@@ -322,7 +322,7 @@ pub fn print_passes() {
 
 pub(crate) fn print(req: PrintRequest, sess: &Session) {
     require_inited();
-    let tm = create_informational_target_machine(sess, true);
+    let tm = create_informational_target_machine(sess);
     unsafe {
         match req {
             PrintRequest::TargetCPUs => llvm::LLVMRustPrintTargetCPUs(tm),

src/librustc_codegen_ssa/back/link.rs

@@ -12,7 +12,7 @@ use rustc_session::search_paths::PathKind;
 /// need out of the shared crate context before we get rid of it.
 use rustc_session::{filesearch, Session};
 use rustc_span::symbol::Symbol;
-use rustc_target::spec::{LinkerFlavor, LldFlavor, PanicStrategy, RelroLevel};
+use rustc_target::spec::{LinkerFlavor, LldFlavor, PanicStrategy, RelocModel, RelroLevel};
 
 use super::archive::ArchiveBuilder;
 use super::command::Command;
@@ -1352,7 +1352,7 @@ fn add_position_independent_executable_args(
     if sess.target.target.options.position_independent_executables {
         let attr_link_args = &*codegen_results.crate_info.link_args;
         let mut user_defined_link_args = sess.opts.cg.link_args.iter().chain(attr_link_args);
-        if is_pic(sess)
+        if sess.relocation_model() == RelocModel::Pic
             && !sess.crt_static(Some(crate_type))
             && !user_defined_link_args.any(|x| x == "-static")
         {
@@ -1992,12 +1992,3 @@ fn are_upstream_rust_objects_already_included(sess: &Session) -> bool {
         config::Lto::No | config::Lto::ThinLocal => false,
     }
 }
-
-fn is_pic(sess: &Session) -> bool {
-    let reloc_model_arg = match sess.opts.cg.relocation_model {
-        Some(ref s) => &s[..],
-        None => &sess.target.target.options.relocation_model[..],
-    };
-
-    reloc_model_arg == "pic"
-}

src/librustc_codegen_ssa/back/write.rs

@@ -1037,7 +1037,7 @@ fn start_executing_work<B: ExtraBackendMethods>(
         regular_module_config: regular_config,
         metadata_module_config: metadata_config,
         allocator_module_config: allocator_config,
-        tm_factory: TargetMachineFactory(backend.target_machine_factory(tcx.sess, ol, false)),
+        tm_factory: TargetMachineFactory(backend.target_machine_factory(tcx.sess, ol)),
         total_cgus,
         msvc_imps_needed: msvc_imps_needed(tcx),
         target_pointer_width: tcx.sess.target.target.target_pointer_width.clone(),

src/librustc_codegen_ssa/traits/backend.rs

@@ -110,14 +110,10 @@ pub trait ExtraBackendMethods: CodegenBackend + WriteBackendMethods + Sized + Se
         tcx: TyCtxt<'_>,
         cgu_name: Symbol,
     ) -> (ModuleCodegen<Self::Module>, u64);
-    // If find_features is true this won't access `sess.crate_types` by assuming
-    // that `is_pie_binary` is false. When we discover LLVM target features
-    // `sess.crate_types` is uninitialized so we cannot access it.
     fn target_machine_factory(
         &self,
         sess: &Session,
         opt_level: config::OptLevel,
-        find_features: bool,
     ) -> Arc<dyn Fn() -> Result<Self::TargetMachine, String> + Send + Sync>;
     fn target_cpu<'b>(&self, sess: &'b Session) -> &'b str;
 }

src/librustc_interface/tests.rs

@@ -14,7 +14,7 @@ use rustc_session::{build_session, Session};
 use rustc_span::edition::{Edition, DEFAULT_EDITION};
 use rustc_span::symbol::sym;
 use rustc_span::SourceFileHashAlgorithm;
-use rustc_target::spec::{LinkerFlavor, MergeFunctions, PanicStrategy, RelroLevel};
+use rustc_target::spec::{LinkerFlavor, MergeFunctions, PanicStrategy, RelocModel, RelroLevel};
 use std::collections::{BTreeMap, BTreeSet};
 use std::iter::FromIterator;
 use std::path::PathBuf;
@@ -430,7 +430,7 @@ fn test_codegen_options_tracking_hash() {
     tracked!(prefer_dynamic, true);
     tracked!(profile_generate, SwitchWithOptPath::Enabled(None));
     tracked!(profile_use, Some(PathBuf::from("abc")));
-    tracked!(relocation_model, Some(String::from("relocation model")));
+    tracked!(relocation_model, Some(RelocModel::Pic));
     tracked!(soft_float, true);
     tracked!(target_cpu, Some(String::from("abc")));
     tracked!(target_feature, String::from("all the features, all of them"));