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.

9 files changed, 17 insertions, 6 deletions

diff --git a/compiler/rustc_codegen_gcc/src/common.rs b/compiler/rustc_codegen_gcc/src/common.rs
index 76fc7bd222e..ac04b61a306 100644
--- a/compiler/rustc_codegen_gcc/src/common.rs
+++ b/compiler/rustc_codegen_gcc/src/common.rs
@@ -73,6 +73,11 @@ impl<'gcc, 'tcx> ConstMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
         }
     }
 
+    fn const_poison(&self, typ: Type<'gcc>) -> RValue<'gcc> {
+        // No distinction between undef and poison.
+        self.const_undef(typ)
+    }
+
     fn const_int(&self, typ: Type<'gcc>, int: i64) -> RValue<'gcc> {
         self.gcc_int(typ, int)
     }
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;
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 13c4fa132d8..74b9ff7eace 100644
--- a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
@@ -295,7 +295,7 @@ 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;
diff --git a/tests/codegen/adjustments.rs b/tests/codegen/adjustments.rs
index 6d224751752..b53a68a5588 100644
--- a/tests/codegen/adjustments.rs
+++ b/tests/codegen/adjustments.rs
@@ -13,7 +13,7 @@ pub fn helper(_: usize) {
 pub fn no_op_slice_adjustment(x: &[u8]) -> &[u8] {
     // We used to generate an extra alloca and memcpy for the block's trailing expression value, so
     // check that we copy directly to the return value slot
-// CHECK: %0 = insertvalue { {{\[0 x i8\]\*|ptr}}, [[USIZE]] } undef, {{\[0 x i8\]\*|ptr}} %x.0, 0
+// CHECK: %0 = insertvalue { {{\[0 x i8\]\*|ptr}}, [[USIZE]] } poison, {{\[0 x i8\]\*|ptr}} %x.0, 0
 // CHECK: %1 = insertvalue { {{\[0 x i8\]\*|ptr}}, [[USIZE]] } %0, [[USIZE]] %x.1, 1
 // CHECK: ret { {{\[0 x i8\]\*|ptr}}, [[USIZE]] } %1
     { x }