Auto merge of #109220 - nikic:poison, r=cuviper

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.

r? `@cuviper`

4 files changed, 5 insertions, 4 deletions

diff --git a/compiler/rustc_codegen_ssa/src/mir/operand.rs b/compiler/rustc_codegen_ssa/src/mir/operand.rs
index 34a5b638d7e..25721f75583 100644
--- a/compiler/rustc_codegen_ssa/src/mir/operand.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/operand.rs
@@ -60,7 +60,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
     ) -> OperandRef<'tcx, V> {
         assert!(layout.is_zst());
         OperandRef {
-            val: OperandValue::Immediate(bx.const_undef(bx.immediate_backend_type(layout))),
+            val: OperandValue::Immediate(bx.const_poison(bx.immediate_backend_type(layout))),
             layout,
         }
     }
@@ -145,7 +145,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
             let llty = bx.cx().backend_type(self.layout);
             debug!("Operand::immediate_or_packed_pair: packing {:?} into {:?}", self, llty);
             // Reconstruct the immediate aggregate.
-            let mut llpair = bx.cx().const_undef(llty);
+            let mut llpair = bx.cx().const_poison(llty);
             let imm_a = bx.from_immediate(a);
             let imm_b = bx.from_immediate(b);
             llpair = bx.insert_value(llpair, imm_a, 0);
diff --git a/compiler/rustc_codegen_ssa/src/mir/place.rs b/compiler/rustc_codegen_ssa/src/mir/place.rs
index cf02f59f67b..f6523a448e3 100644
--- a/compiler/rustc_codegen_ssa/src/mir/place.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/place.rs
@@ -214,7 +214,7 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
         let cast_to_size = cast_to_layout.layout.size();
         let cast_to = bx.cx().immediate_backend_type(cast_to_layout);
         if self.layout.abi.is_uninhabited() {
-            return bx.cx().const_undef(cast_to);
+            return bx.cx().const_poison(cast_to);
         }
         let (tag_scalar, tag_encoding, tag_field) = match self.layout.variants {
             Variants::Single { index } => {
diff --git a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs b/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
index 72d41d8c32c..14f57b63e97 100644
--- a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
@@ -346,7 +346,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         assert!(bx.cx().is_backend_immediate(cast));
                         let ll_t_out = bx.cx().immediate_backend_type(cast);
                         if operand.layout.abi.is_uninhabited() {
-                            let val = OperandValue::Immediate(bx.cx().const_undef(ll_t_out));
+                            let val = OperandValue::Immediate(bx.cx().const_poison(ll_t_out));
                             return OperandRef { val, layout: cast };
                         }
                         let r_t_in =
diff --git a/compiler/rustc_codegen_ssa/src/traits/consts.rs b/compiler/rustc_codegen_ssa/src/traits/consts.rs
index fdc7a30e841..61906302779 100644
--- a/compiler/rustc_codegen_ssa/src/traits/consts.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/consts.rs
@@ -8,6 +8,7 @@ pub trait ConstMethods<'tcx>: BackendTypes {
     // Constant constructors
     fn const_null(&self, t: Self::Type) -> Self::Value;
     fn const_undef(&self, t: Self::Type) -> Self::Value;
+    fn const_poison(&self, t: Self::Type) -> Self::Value;
     fn const_int(&self, t: Self::Type, i: i64) -> Self::Value;
     fn const_uint(&self, t: Self::Type, i: u64) -> Self::Value;
     fn const_uint_big(&self, t: Self::Type, u: u128) -> Self::Value;