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rustc_codegen_llvm: Filter out unavailable LLVM features
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Convert to_llvm_features to return Option<LLVMFeature> so that it can
return None if the requested feature is not available for the current
LLVM version.

Add match rules to filter out aarch64 features not available in LLVM 17.
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@mrkajetanp
mrkajetanp committed Aug 27, 2024
1 parent 4fc4019 commit 3a0fbb5
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Showing 2 changed files with 63 additions and 38 deletions.
7 changes: 4 additions & 3 deletions compiler/rustc_codegen_llvm/src/attributes.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -521,9 +521,10 @@ pub(crate) fn llfn_attrs_from_instance<'ll, 'tcx>(

let function_features = function_features
.iter()
.flat_map(|feat| {
llvm_util::to_llvm_features(cx.tcx.sess, feat).into_iter().map(|f| format!("+{f}"))
})
// Convert to LLVMFeatures and filter out unavailable ones
.flat_map(|feat| llvm_util::to_llvm_features(cx.tcx.sess, feat))
// Convert LLVMFeatures & dependencies to +<feats>s
.flat_map(|feat| feat.into_iter().map(|f| format!("+{f}")))
.chain(codegen_fn_attrs.instruction_set.iter().map(|x| match x {
InstructionSetAttr::ArmA32 => "-thumb-mode".to_string(),
InstructionSetAttr::ArmT32 => "+thumb-mode".to_string(),
Expand Down
94 changes: 59 additions & 35 deletions compiler/rustc_codegen_llvm/src/llvm_util.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -209,7 +209,7 @@ impl<'a> IntoIterator for LLVMFeature<'a> {
// Though note that Rust can also be build with an external precompiled version of LLVM
// which might lead to failures if the oldest tested / supported LLVM version
// doesn't yet support the relevant intrinsics
pub(crate) fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> LLVMFeature<'a> {
pub(crate) fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> Option<LLVMFeature<'a>> {
let arch = if sess.target.arch == "x86_64" {
"x86"
} else if sess.target.arch == "arm64ec" {
Expand All @@ -218,42 +218,59 @@ pub(crate) fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> LLVMFeature<'a
&*sess.target.arch
};
match (arch, s) {
("x86", "sse4.2") => {
LLVMFeature::with_dependency("sse4.2", TargetFeatureFoldStrength::EnableOnly("crc32"))
}
("x86", "pclmulqdq") => LLVMFeature::new("pclmul"),
("x86", "rdrand") => LLVMFeature::new("rdrnd"),
("x86", "bmi1") => LLVMFeature::new("bmi"),
("x86", "cmpxchg16b") => LLVMFeature::new("cx16"),
("x86", "lahfsahf") => LLVMFeature::new("sahf"),
("aarch64", "rcpc2") => LLVMFeature::new("rcpc-immo"),
("aarch64", "dpb") => LLVMFeature::new("ccpp"),
("aarch64", "dpb2") => LLVMFeature::new("ccdp"),
("aarch64", "frintts") => LLVMFeature::new("fptoint"),
("aarch64", "fcma") => LLVMFeature::new("complxnum"),
("aarch64", "pmuv3") => LLVMFeature::new("perfmon"),
("aarch64", "paca") => LLVMFeature::new("pauth"),
("aarch64", "pacg") => LLVMFeature::new("pauth"),
("aarch64", "sve-b16b16") => LLVMFeature::new("b16b16"),
("aarch64", "flagm2") => LLVMFeature::new("altnzcv"),
("x86", "sse4.2") => Some(LLVMFeature::with_dependency(
"sse4.2",
TargetFeatureFoldStrength::EnableOnly("crc32"),
)),
("x86", "pclmulqdq") => Some(LLVMFeature::new("pclmul")),
("x86", "rdrand") => Some(LLVMFeature::new("rdrnd")),
("x86", "bmi1") => Some(LLVMFeature::new("bmi")),
("x86", "cmpxchg16b") => Some(LLVMFeature::new("cx16")),
("x86", "lahfsahf") => Some(LLVMFeature::new("sahf")),
("aarch64", "rcpc2") => Some(LLVMFeature::new("rcpc-immo")),
("aarch64", "dpb") => Some(LLVMFeature::new("ccpp")),
("aarch64", "dpb2") => Some(LLVMFeature::new("ccdp")),
("aarch64", "frintts") => Some(LLVMFeature::new("fptoint")),
("aarch64", "fcma") => Some(LLVMFeature::new("complxnum")),
("aarch64", "pmuv3") => Some(LLVMFeature::new("perfmon")),
("aarch64", "paca") => Some(LLVMFeature::new("pauth")),
("aarch64", "pacg") => Some(LLVMFeature::new("pauth")),
("aarch64", "sve-b16b16") => Some(LLVMFeature::new("b16b16")),
("aarch64", "flagm2") => Some(LLVMFeature::new("altnzcv")),
// Rust ties fp and neon together.
("aarch64", "neon") => {
LLVMFeature::with_dependency("neon", TargetFeatureFoldStrength::Both("fp-armv8"))
Some(LLVMFeature::with_dependency("neon", TargetFeatureFoldStrength::Both("fp-armv8")))
}
// In LLVM neon implicitly enables fp, but we manually enable
// neon when a feature only implicitly enables fp
("aarch64", "fhm") => LLVMFeature::new("fp16fml"),
("aarch64", "fp16") => LLVMFeature::new("fullfp16"),
("aarch64", "fhm") => Some(LLVMFeature::new("fp16fml")),
("aarch64", "fp16") => Some(LLVMFeature::new("fullfp16")),
// Filter out features that are not supported by the current LLVM version
("aarch64", "faminmax") if get_version().0 < 18 => None,
("aarch64", "fp8") if get_version().0 < 18 => None,
("aarch64", "fp8dot2") if get_version().0 < 18 => None,
("aarch64", "fp8dot4") if get_version().0 < 18 => None,
("aarch64", "fp8fma") if get_version().0 < 18 => None,
("aarch64", "fpmr") if get_version().0 != 18 => None,
("aarch64", "lut") if get_version().0 < 18 => None,
("aarch64", "sme-f8f16") if get_version().0 < 18 => None,
("aarch64", "sme-f8f32") if get_version().0 < 18 => None,
("aarch64", "sme-fa64") if get_version().0 < 18 => None,
("aarch64", "sme-lutv2") if get_version().0 < 18 => None,
("aarch64", "ssve-fp8dot2") if get_version().0 < 18 => None,
("aarch64", "ssve-fp8dot4") if get_version().0 < 18 => None,
("aarch64", "ssve-fp8fma") if get_version().0 < 18 => None,
("aarch64", "v9.5a") if get_version().0 < 18 => None,
// In LLVM 18, `unaligned-scalar-mem` was merged with `unaligned-vector-mem` into a single feature called
// `fast-unaligned-access`. In LLVM 19, it was split back out.
("riscv32" | "riscv64", "unaligned-scalar-mem") if get_version().0 == 18 => {
LLVMFeature::new("fast-unaligned-access")
Some(LLVMFeature::new("fast-unaligned-access"))
}
// For LLVM 18, enable the evex512 target feature if a avx512 target feature is enabled.
("x86", s) if get_version().0 >= 18 && s.starts_with("avx512") => {
LLVMFeature::with_dependency(s, TargetFeatureFoldStrength::EnableOnly("evex512"))
Some(LLVMFeature::with_dependency(s, TargetFeatureFoldStrength::EnableOnly("evex512")))
}
(_, s) => LLVMFeature::new(s),
(_, s) => Some(LLVMFeature::new(s)),
}
}

Expand Down Expand Up @@ -293,13 +310,17 @@ pub fn target_features(sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
return true;
}
// check that all features in a given smallvec are enabled
for llvm_feature in to_llvm_features(sess, feature) {
let cstr = SmallCStr::new(llvm_feature);
if !unsafe { llvm::LLVMRustHasFeature(&target_machine, cstr.as_ptr()) } {
return false;
if let Some(feat) = to_llvm_features(sess, feature) {
for llvm_feature in feat {
let cstr = SmallCStr::new(llvm_feature);
if !unsafe { llvm::LLVMRustHasFeature(&target_machine, cstr.as_ptr()) } {
return false;
}
}
true
} else {
false
}
true
})
.map(|(feature, _, _)| Symbol::intern(feature)),
);
Expand Down Expand Up @@ -388,9 +409,9 @@ fn print_target_features(out: &mut String, sess: &Session, tm: &llvm::TargetMach
.target
.supported_target_features()
.iter()
.map(|(feature, _gate, _implied)| {
.filter_map(|(feature, _gate, _implied)| {
// LLVM asserts that these are sorted. LLVM and Rust both use byte comparison for these strings.
let llvm_feature = to_llvm_features(sess, *feature).llvm_feature_name;
let llvm_feature = to_llvm_features(sess, *feature)?.llvm_feature_name;
let desc =
match llvm_target_features.binary_search_by_key(&llvm_feature, |(f, _d)| f).ok() {
Some(index) => {
Expand All @@ -400,7 +421,7 @@ fn print_target_features(out: &mut String, sess: &Session, tm: &llvm::TargetMach
None => "",
};

(*feature, desc)
Some((*feature, desc))
})
.collect::<Vec<_>>();

Expand Down Expand Up @@ -597,7 +618,7 @@ pub(crate) fn global_llvm_features(
if feature_state.is_none() {
let rust_feature =
supported_features.iter().find_map(|&(rust_feature, _, _)| {
let llvm_features = to_llvm_features(sess, rust_feature);
let llvm_features = to_llvm_features(sess, rust_feature)?;
if llvm_features.contains(feature)
&& !llvm_features.contains(rust_feature)
{
Expand Down Expand Up @@ -643,7 +664,7 @@ pub(crate) fn global_llvm_features(
// passing requests down to LLVM. This means that all in-language
// features also work on the command line instead of having two
// different names when the LLVM name and the Rust name differ.
let llvm_feature = to_llvm_features(sess, feature);
let llvm_feature = to_llvm_features(sess, feature)?;

Some(
std::iter::once(format!(
Expand Down Expand Up @@ -693,6 +714,9 @@ fn backend_feature_name<'a>(sess: &Session, s: &'a str) -> Option<&'a str> {
let feature = s
.strip_prefix(&['+', '-'][..])
.unwrap_or_else(|| sess.dcx().emit_fatal(InvalidTargetFeaturePrefix { feature: s }));
if s.is_empty() {
return None;
}
// Rustc-specific feature requests like `+crt-static` or `-crt-static`
// are not passed down to LLVM.
if RUSTC_SPECIFIC_FEATURES.contains(&feature) {
Expand Down

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