implement -Zllvm-target-feature

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -100,14 +100,17 @@ fn write_output_file<'ll>(
     result.into_result().unwrap_or_else(|()| llvm_err(dcx, LlvmError::WriteOutput { path: output }))
 }
 
+/// If `for_cfg` is `true` then we are creating this machine for the purpose of populating
+/// [`rustc_codegen_ssa::TargetConfig`] based on what LLVM actually enables in this configuration.
+/// `-Ctarget-feature` should be ignored in that case since it is already processed separately.
 pub(crate) fn create_informational_target_machine(
     sess: &Session,
-    only_base_features: bool,
+    for_cfg: bool,
 ) -> OwnedTargetMachine {
     let config = TargetMachineFactoryConfig { split_dwarf_file: None, output_obj_file: None };
     // Can't use query system here quite yet because this function is invoked before the query
     // system/tcx is set up.
-    let features = llvm_util::global_llvm_features(sess, only_base_features);
+    let features = llvm_util::global_llvm_features(sess, for_cfg);
     target_machine_factory(sess, config::OptLevel::No, &features)(sess.dcx(), config)
 }
 

compiler/rustc_codegen_llvm/src/diagnostics.rs

@@ -245,3 +245,10 @@ pub(crate) struct IntrinsicWrongArch<'a> {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag("ignoring feature with missing prefix in `-Zllvm-target-feature`: `{$feature}`")]
+#[note("features must begin with a `+` to enable or `-` to disable it")]
+pub(crate) struct UnknownLlvmTargetFeaturePrefix<'a> {
+    pub feature: &'a str,
+}

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -646,7 +646,11 @@ fn llvm_features_by_flags(sess: &Session, features: &mut Vec<String>) {
 
 /// The list of LLVM features computed from CLI flags (`-Ctarget-cpu`, `-Ctarget-feature`,
 /// `--target` and similar).
-pub(crate) fn global_llvm_features(sess: &Session, only_base_features: bool) -> Vec<String> {
+///
+/// If `for_cfg` is `true` then we are assembling the feature list for the purpose of populating
+/// [`rustc_codegen_ssa::TargetConfig`] based on what LLVM actually enables in this configuration.
+/// `-Ctarget-feature` should be ignored in that case since it is already processed separately.
+pub(crate) fn global_llvm_features(sess: &Session, for_cfg: bool) -> Vec<String> {
     // Features that come earlier are overridden by conflicting features later in the string.
     // Typically we'll want more explicit settings to override the implicit ones, so:
     //
@@ -725,14 +729,33 @@ pub(crate) fn global_llvm_features(sess: &Session, only_base_features: bool) ->
     // Features implied by an implicit or explicit `--target`.
     target_features::target_spec_to_backend_features(sess, &mut extend_backend_features);
 
-    // -Ctarget-features
-    if !only_base_features {
+    // -Ctarget-features. Skipped for `cfg` as there we parse -Ctarget-features directly instead of
+    // going via an LLVM target machine (which avoids accidentally picking up LLVM-level target
+    // feature implications that we do not want).
+    if !for_cfg {
         target_features::flag_to_backend_features(sess, extend_backend_features);
     }
 
     // We add this in the "base target" so that these show up in `sess.unstable_target_features`.
     llvm_features_by_flags(sess, &mut features);
 
+    // `-Zllvm-target-features`, all the way at the end to overwrite everything.
+    // Should be picked up by `cfg` (e.g. if someone enables AVX this way).
+    for feature in sess.opts.unstable_opts.llvm_target_feature.split(',') {
+        if feature.is_empty() {
+            continue;
+        }
+        if feature.starts_with('+') || feature.starts_with('-') {
+            features.push(feature.to_owned());
+        } else {
+            // LLVM seems to silently ignore entries without leading `+`/`-`. Let's emit a warning
+            // to avoid confusion. But only emit this warning once, under `for_cfg`.
+            if for_cfg {
+                sess.dcx().emit_warn(diagnostics::UnknownLlvmTargetFeaturePrefix { feature });
+            }
+        }
+    }
+
     features
 }
 

compiler/rustc_codegen_ssa/src/diagnostics.rs

@@ -1198,7 +1198,7 @@ pub(crate) struct XcrunSdkPathWarning {
 pub(crate) struct Aarch64SoftfloatNeon;
 
 #[derive(Diagnostic)]
-#[diag("unknown feature specified for `-Ctarget-feature`: `{$feature}`")]
+#[diag("ignoring feature with missing prefix in `-Ctarget-feature`: `{$feature}`")]
 #[note("features must begin with a `+` to enable or `-` to disable it")]
 pub(crate) struct UnknownCTargetFeaturePrefix<'a> {
     pub feature: &'a str,

compiler/rustc_session/src/options.rs

@@ -2582,6 +2582,10 @@ options! {
         "a list of module flags to pass to LLVM (space separated)"),
     llvm_plugins: Vec<String> = (Vec::new(), parse_list, [TRACKED],
         "a list LLVM plugins to enable (space separated)"),
+    llvm_target_feature: String = (String::new(), parse_target_feature, [TRACKED] { TARGET_MODIFIER: LlvmTargetFeature },
+        "enable/disable LLVM-level target features. \
+        This feature is unsafe and can cause ABI issues and compiler crashes, \
+        because LLVM does not support all target feature combinations."),
     llvm_time_trace: bool = (false, parse_bool, [UNTRACKED],
         "generate JSON tracing data file from LLVM data (default: no)"),
     llvm_writable: bool = (false, parse_bool, [TRACKED],

tests/ui/target-feature/llvm-target-feature.rs

@@ -0,0 +1,32 @@
+//! Sometimes `-Ctarget-cpu` can *disable* target features that would by default be enabled on the
+//! current target. Ensure that we catch the case where those target features are important for the
+//! ABI.
+
+//@ compile-flags: --crate-type=lib
+//@ compile-flags: --target=x86_64-unknown-linux-gnu
+//@ compile-flags: -Zllvm-target-feature=+avx2
+//@ needs-llvm-components: x86
+
+//@ build-pass
+//@ ignore-backends: gcc
+//@ add-minicore
+
+#![feature(no_core, intrinsics, rustc_attrs)]
+#![no_core]
+#![allow(improper_ctypes_definitions)]
+
+extern crate minicore;
+use minicore::*;
+
+// Also test the ABI checks by using `extern "C"`
+#[no_mangle] // force codegen
+pub extern "C" fn do_thing(x: simd::f32x8, y: simd::f32x8) -> simd::f32x8 {
+    #[rustc_intrinsic]
+    #[rustc_nounwind]
+    pub const unsafe fn simd_add<T>(x: T, y: T) -> T;
+
+    unsafe { simd_add(x, y) }
+}
+
+#[cfg(not(target_feature = "avx2"))]
+compile_error!("the avx2 cfg did not get set");

tests/ui/target-feature/missing-plusminus-2.rs

@@ -5,4 +5,4 @@
 #![feature(no_core)]
 #![no_core]
 
-//~? WARN unknown feature specified for `-Ctarget-feature`: `rdrand`
+//~? WARN ignoring feature with missing prefix in `-Ctarget-feature`: `rdrand`

tests/ui/target-feature/missing-plusminus-2.stderr

@@ -1,4 +1,4 @@
-warning: unknown feature specified for `-Ctarget-feature`: `rdrand`
+warning: ignoring feature with missing prefix in `-Ctarget-feature`: `rdrand`
    |
    = note: features must begin with a `+` to enable or `-` to disable it
 

tests/ui/target-feature/missing-plusminus-llvm.rs

@@ -0,0 +1,13 @@
+//@ compile-flags: -Zllvm-target-feature=banana --crate-type=rlib
+//@ build-pass
+
+//@ ignore-backends: gcc
+//@ add-minicore
+
+#![feature(no_core, intrinsics, rustc_attrs)]
+#![no_core]
+
+extern crate minicore;
+use minicore::*;
+
+//~? WARN ignoring feature with missing prefix in `-Zllvm-target-feature`: `banana`

tests/ui/target-feature/missing-plusminus-llvm.stderr

@@ -0,0 +1,6 @@
+warning: ignoring feature with missing prefix in `-Zllvm-target-feature`: `banana`
+   |
+   = note: features must begin with a `+` to enable or `-` to disable it
+
+warning: 1 warning emitted
+

tests/ui/target-feature/missing-plusminus.rs

@@ -1,4 +1,4 @@
 //@ compile-flags: -Ctarget-feature=banana --crate-type=rlib
 //@ build-pass
 
-//~? WARN unknown feature specified for `-Ctarget-feature`: `banana`
+//~? WARN ignoring feature with missing prefix in `-Ctarget-feature`: `banana`

tests/ui/target-feature/missing-plusminus.stderr

@@ -1,4 +1,4 @@
-warning: unknown feature specified for `-Ctarget-feature`: `banana`
+warning: ignoring feature with missing prefix in `-Ctarget-feature`: `banana`
    |
    = note: features must begin with a `+` to enable or `-` to disable it