Default to -Z plt=yes

6009da0 defaulted to `-Z plt=no` (like
`clang -fno-plt`) which a not a useful default[1].

On x86-64, if the target symbol is preemptible, there is an
`R_X86_64_GLOB_DAT` relocation, and the (very minor) optimization works as
intended. However, if the target is non-preemptible, i.e. the target is
resolved to the same component, this is actually a pessimization due to
the longer instruction.

On RISC architectures, there is typically no single instruction which
can load a GOT entry and perform an indirect call. `-fno-plt` has a longer
code quence. For example, AArch64 needs 3 instructions:

    adrp    x0, _GLOBAL_OFFSET_TABLE_
    ldr     x0, [x0, #:gotpage_lo15:bar]
    br      x0

This does not end up with a serious code size issue, because LLVM
"RtLibUseGOT" is not implemented for non-x86 targets.

On x86-32, very new lld[2] (2022-12-31) is needed to support
general-dynamic/local-dynamic TLS models.

`-Z plt=no` is not an appropriate default, so just default to true for
all targets.

[1] https://maskray.me/blog/2021-09-19-all-about-procedure-linkage-table#fno-plt
[2] llvm/llvm-project@8dc7366

Author: MaskRay

compiler/rustc_interface/src/tests.rs

@@ -766,7 +766,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(panic_abort_tests, true);
     tracked!(panic_in_drop, PanicStrategy::Abort);
     tracked!(pick_stable_methods_before_any_unstable, false);
-    tracked!(plt, Some(true));
+    tracked!(plt, false);
     tracked!(polonius, true);
     tracked!(precise_enum_drop_elaboration, false);
     tracked!(print_fuel, Some("abc".to_string()));

compiler/rustc_session/src/options.rs

@@ -1479,10 +1479,10 @@ options! {
         "print some performance-related statistics (default: no)"),
     pick_stable_methods_before_any_unstable: bool = (true, parse_bool, [TRACKED],
         "try to pick stable methods first before picking any unstable methods (default: yes)"),
-    plt: Option<bool> = (None, parse_opt_bool, [TRACKED],
-        "whether to use the PLT when calling into shared libraries;
-        only has effect for PIC code on systems with ELF binaries
-        (default: PLT is disabled if full relro is enabled)"),
+    plt: bool = (true, parse_bool, [TRACKED],
+        "if false, use GOT-generating code sequences for external function calls. \
+        This results in longer code sequences, but may avoid a PLT if the function is not bound locally. \
+        Only has effect on ELF systems (default: yes)"),
     polonius: bool = (false, parse_bool, [TRACKED],
         "enable polonius-based borrow-checker (default: no)"),
     polymorphize: bool = (false, parse_bool, [TRACKED],

compiler/rustc_session/src/session.rs

@@ -37,7 +37,7 @@ use rustc_span::edition::Edition;
 use rustc_span::source_map::{FileLoader, RealFileLoader, SourceMap, Span};
 use rustc_span::{sym, SourceFileHashAlgorithm, Symbol};
 use rustc_target::asm::InlineAsmArch;
-use rustc_target::spec::{CodeModel, PanicStrategy, RelocModel, RelroLevel};
+use rustc_target::spec::{CodeModel, PanicStrategy, RelocModel};
 use rustc_target::spec::{
     DebuginfoKind, SanitizerSet, SplitDebuginfo, StackProtector, Target, TargetTriple, TlsModel,
 };
@@ -908,22 +908,7 @@ impl Session {
 
     /// Returns `true` if we cannot skip the PLT for shared library calls.
     pub fn needs_plt(&self) -> bool {
-        // Check if the current target usually needs PLT to be enabled.
-        // The user can use the command line flag to override it.
-        let needs_plt = self.target.needs_plt;
-
-        let dbg_opts = &self.opts.unstable_opts;
-
-        let relro_level = dbg_opts.relro_level.unwrap_or(self.target.relro_level);
-
-        // Only enable this optimization by default if full relro is also enabled.
-        // In this case, lazy binding was already unavailable, so nothing is lost.
-        // This also ensures `-Wl,-z,now` is supported by the linker.
-        let full_relro = RelroLevel::Full == relro_level;
-
-        // If user didn't explicitly forced us to use / skip the PLT,
-        // then try to skip it where possible.
-        dbg_opts.plt.unwrap_or(needs_plt || !full_relro)
+        self.opts.unstable_opts.plt
     }
 
     /// Checks if LLVM lifetime markers should be emitted.

compiler/rustc_target/src/spec/mod.rs

@@ -1549,9 +1549,6 @@ pub struct TargetOptions {
     pub position_independent_executables: bool,
     /// Executables that are both statically linked and position-independent are supported.
     pub static_position_independent_executables: bool,
-    /// Determines if the target always requires using the PLT for indirect
-    /// library calls or not. This controls the default value of the `-Z plt` flag.
-    pub needs_plt: bool,
     /// Either partial, full, or off. Full RELRO makes the dynamic linker
     /// resolve all symbols at startup and marks the GOT read-only before
     /// starting the program, preventing overwriting the GOT.
@@ -1871,7 +1868,6 @@ impl Default for TargetOptions {
             no_default_libraries: true,
             position_independent_executables: false,
             static_position_independent_executables: false,
-            needs_plt: false,
             relro_level: RelroLevel::None,
             pre_link_objects: Default::default(),
             post_link_objects: Default::default(),
@@ -2544,7 +2540,6 @@ impl Target {
         key!(no_default_libraries, bool);
         key!(position_independent_executables, bool);
         key!(static_position_independent_executables, bool);
-        key!(needs_plt, bool);
         key!(relro_level, RelroLevel)?;
         key!(archive_format);
         key!(allow_asm, bool);
@@ -2798,7 +2793,6 @@ impl ToJson for Target {
         target_option_val!(no_default_libraries);
         target_option_val!(position_independent_executables);
         target_option_val!(static_position_independent_executables);
-        target_option_val!(needs_plt);
         target_option_val!(relro_level);
         target_option_val!(archive_format);
         target_option_val!(allow_asm);

compiler/rustc_target/src/spec/x86_64_unknown_linux_gnux32.rs

@@ -8,9 +8,6 @@ pub fn target() -> Target {
     base.add_pre_link_args(LinkerFlavor::Gnu(Cc::Yes, Lld::No), &["-mx32"]);
     base.stack_probes = StackProbeType::X86;
     base.has_thread_local = false;
-    // BUG(GabrielMajeri): disabling the PLT on x86_64 Linux with x32 ABI
-    // breaks code gen. See LLVM bug 36743
-    base.needs_plt = true;
 
     Target {
         llvm_target: "x86_64-unknown-linux-gnux32".into(),

tests/assembly/pic-relocation-model.rs

@@ -14,7 +14,7 @@ trait Sized {}
 trait Copy {}
 
 // CHECK-LABEL: call_other_fn:
-// CHECK:       {{(jmpq|callq)}} *other_fn@GOTPCREL(%rip)
+// CHECK:       {{(jmp|callq)}} other_fn@PLT
 #[no_mangle]
 pub fn call_other_fn() -> u8 {
     unsafe {
@@ -23,7 +23,7 @@ pub fn call_other_fn() -> u8 {
 }
 
 // CHECK-LABEL: other_fn:
-// CHECK:       callq *foreign_fn@GOTPCREL(%rip)
+// CHECK:       callq foreign_fn@PLT
 #[no_mangle]
 #[inline(never)]
 pub fn other_fn() -> u8 {

tests/assembly/pie-relocation-model.rs

@@ -24,9 +24,7 @@ pub fn call_other_fn() -> u8 {
 }
 
 // CHECK-LABEL: other_fn:
-// External functions are still called through GOT, since we don't know if the symbol
-// is defined in the binary or in the shared library.
-// CHECK:       callq *foreign_fn@GOTPCREL(%rip)
+// CHECK:       callq foreign_fn@PLT
 #[no_mangle]
 #[inline(never)]
 pub fn other_fn() -> u8 {

tests/rustdoc-ui/z-help.stdout

@@ -107,9 +107,7 @@
     -Z                              parse-only=val -- parse only; do not compile, assemble, or link (default: no)
     -Z                              perf-stats=val -- print some performance-related statistics (default: no)
     -Z pick-stable-methods-before-any-unstable=val -- try to pick stable methods first before picking any unstable methods (default: yes)
-    -Z                                     plt=val -- whether to use the PLT when calling into shared libraries;
-        only has effect for PIC code on systems with ELF binaries
-        (default: PLT is disabled if full relro is enabled)
+    -Z                                     plt=val -- if false, use GOT-generating code sequences for external function calls. This results in longer code sequences, but may avoid a PLT if the function is not bound locally. Only has effect on ELF systems (default: yes)
     -Z                                polonius=val -- enable polonius-based borrow-checker (default: no)
     -Z                            polymorphize=val -- perform polymorphization analysis
     -Z                            pre-link-arg=val -- a single extra argument to prepend the linker invocation (can be used several times)