Implicitly enable evex512 if avx512 is enabled

LLVM 18 requires the evex512 feature to allow use of zmm registers.
LLVM automatically sets it when using a generic CPU, but not when
`-C target-cpu` is specified. This will result either in backend
legalization crashes, or code unexpectedly using ymm instead of
zmm registers.

For now, make sure that `avx512*` features imply `evex512`. Long
term we'll probably have to deal with the AVX10 mess somehow.

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -266,6 +266,10 @@ pub fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> LLVMFeature<'a> {
         ("riscv32" | "riscv64", "fast-unaligned-access") if get_version().0 <= 17 => {
             LLVMFeature::new("unaligned-scalar-mem")
         }
+        // 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"))
+        }
         (_, s) => LLVMFeature::new(s),
     }
 }

tests/ui/asm/x86_64/evex512-implicit-feature.rs

@@ -0,0 +1,15 @@
+// build-pass
+// only-x86_64
+// compile-flags: --crate-type=lib -C target-cpu=skylake
+
+#![feature(avx512_target_feature)]
+#![feature(stdarch_x86_avx512)]
+
+use std::arch::x86_64::*;
+
+#[target_feature(enable = "avx512f")]
+#[no_mangle]
+pub unsafe fn test(res: *mut f64, p: *const f64) {
+    let arg = _mm512_load_pd(p);
+    _mm512_store_pd(res, _mm512_fmaddsub_pd(arg, arg, arg));
+}