Use poison instead of undef

In cases where it is legal, we should prefer poison values over
undef values.

This replaces undef with poison for aggregate construction and
for uninhabited types. There are more places where we can likely
use poison, but I wanted to stay conservative to start with.

In particular the aggregate case is important for newer LLVM
versions, which are not able to handle an undef base value during
early optimization due to poison-propagation concerns.

3 files changed, 6 insertions, 1 deletions

diff --git a/compiler/rustc_codegen_llvm/src/builder.rs b/compiler/rustc_codegen_llvm/src/builder.rs
index 0f33b985489..580451ba265 100644
--- a/compiler/rustc_codegen_llvm/src/builder.rs
+++ b/compiler/rustc_codegen_llvm/src/builder.rs
@@ -990,7 +990,7 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
 
     fn resume(&mut self, exn0: &'ll Value, exn1: &'ll Value) {
         let ty = self.type_struct(&[self.type_i8p(), self.type_i32()], false);
-        let mut exn = self.const_undef(ty);
+        let mut exn = self.const_poison(ty);
         exn = self.insert_value(exn, exn0, 0);
         exn = self.insert_value(exn, exn1, 1);
         unsafe {
diff --git a/compiler/rustc_codegen_llvm/src/common.rs b/compiler/rustc_codegen_llvm/src/common.rs
index b0a9a30ab46..efa0c13226e 100644
--- a/compiler/rustc_codegen_llvm/src/common.rs
+++ b/compiler/rustc_codegen_llvm/src/common.rs
@@ -130,6 +130,10 @@ impl<'ll, 'tcx> ConstMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         unsafe { llvm::LLVMGetUndef(t) }
     }
 
+    fn const_poison(&self, t: &'ll Type) -> &'ll Value {
+        unsafe { llvm::LLVMGetPoison(t) }
+    }
+
     fn const_int(&self, t: &'ll Type, i: i64) -> &'ll Value {
         unsafe { llvm::LLVMConstInt(t, i as u64, True) }
     }
diff --git a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
index 509cb0fef56..9e5265188b5 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
@@ -1069,6 +1069,7 @@ extern "C" {
     // Operations on constants of any type
     pub fn LLVMConstNull(Ty: &Type) -> &Value;
     pub fn LLVMGetUndef(Ty: &Type) -> &Value;
+    pub fn LLVMGetPoison(Ty: &Type) -> &Value;
 
     // Operations on metadata
     pub fn LLVMMDStringInContext(C: &Context, Str: *const c_char, SLen: c_uint) -> &Value;