fix(rustc_codegen_nvvm): Check function signatures before overriding libm functions. (Rust-GPU#206)

libm 0.2.13 refactored some of its implementations to use generic functions. The monomorphized function names clashed with the function names the codegen would override, causing it the codegen to override the function to a libdevice call that did not match the function signature. It then tries to patch this with a bitcast betwen floating point types, which are invalid casts.

This change adds an additional check before overriding. The libm function signature must match the libdevice intrinsic signature of the same name. Note that this prevents overrides even if bitcasts between the two argument types would be valid, which is likely another bug anyways.

Author: jorge-ortega

crates/rustc_codegen_nvvm/src/override_fns.rs

@@ -2,11 +2,16 @@
 //! codegen-builtin methods. Currently the only use for this is overriding libm functions
 //! with libdevice intrinsics (which are much faster and smaller).
 
-use crate::{builder::Builder, context::CodegenCx, llvm};
+use crate::abi::FnAbiLlvmExt;
+use crate::builder::Builder;
+use crate::context::CodegenCx;
+use crate::llvm;
 use rustc_codegen_ssa::mono_item::MonoItemExt;
-use rustc_codegen_ssa::traits::BuilderMethods;
+use rustc_codegen_ssa::traits::{BaseTypeCodegenMethods, BuilderMethods};
 use rustc_hir::def_id::LOCAL_CRATE;
-use rustc_middle::{mir::mono::MonoItem, ty::Instance};
+use rustc_middle::mir::mono::MonoItem;
+use rustc_middle::ty::layout::FnAbiOf;
+use rustc_middle::ty::{self, Instance};
 
 /// Either override or define a function.
 pub(crate) fn define_or_override_fn<'tcx>(func: Instance<'tcx>, cx: &CodegenCx<'_, 'tcx>) {
@@ -27,12 +32,24 @@ fn should_override<'tcx>(func: Instance<'tcx>, cx: &CodegenCx<'_, 'tcx>) -> bool
     if !is_libm {
         return false;
     }
+
     let sym = cx.tcx.item_name(func.def_id());
     let name = sym.as_str();
-    let intrinsics = cx.intrinsics_map.borrow();
-    let is_known_intrinsic = intrinsics.contains_key(format!("__nv_{}", name).as_str());
 
-    !is_unsupported_libdevice_fn(name) && is_known_intrinsic
+    if is_unsupported_libdevice_fn(name) {
+        return false;
+    }
+
+    let libdevice_name = format!("__nv_{}", name);
+    let ld_fn = if let Some((args, ret)) = cx.intrinsics_map.borrow().get(libdevice_name.as_str()) {
+        cx.type_func(args, ret)
+    } else {
+        return false;
+    };
+
+    // Check the function signatures match.
+    let lm_fn = cx.fn_abi_of_instance(func, ty::List::empty()).llvm_type(cx);
+    lm_fn == ld_fn
 }
 
 fn is_unsupported_libdevice_fn(name: &str) -> bool {