Auto merge of #137058 - scottmcm:trunc-unchecked, r=nikic

Emit `trunc nuw` for unchecked shifts and `to_immediate_scalar`

- For shifts this shrinks the IR by no longer needing an `assume` while still providing the UB information
- Having this on the `i8`→`i1` truncations will hopefully help with some places that have to load `i8`s or pass those in LLVM structs without range information

2 files changed, 17 insertions, 8 deletions

diff --git a/compiler/rustc_codegen_gcc/src/builder.rs b/compiler/rustc_codegen_gcc/src/builder.rs
index b8e37b60480..c8b7616e645 100644
--- a/compiler/rustc_codegen_gcc/src/builder.rs
+++ b/compiler/rustc_codegen_gcc/src/builder.rs
@@ -989,10 +989,14 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> {
             OperandValue::Ref(place.val)
         } else if place.layout.is_gcc_immediate() {
             let load = self.load(place.layout.gcc_type(self), place.val.llval, place.val.align);
-            if let abi::BackendRepr::Scalar(ref scalar) = place.layout.backend_repr {
-                scalar_load_metadata(self, load, scalar);
-            }
-            OperandValue::Immediate(self.to_immediate(load, place.layout))
+            OperandValue::Immediate(
+                if let abi::BackendRepr::Scalar(ref scalar) = place.layout.backend_repr {
+                    scalar_load_metadata(self, load, scalar);
+                    self.to_immediate_scalar(load, *scalar)
+                } else {
+                    load
+                },
+            )
         } else if let abi::BackendRepr::ScalarPair(ref a, ref b) = place.layout.backend_repr {
             let b_offset = a.size(self).align_to(b.align(self).abi);
 
@@ -1694,7 +1698,7 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> {
 
     fn to_immediate_scalar(&mut self, val: Self::Value, scalar: abi::Scalar) -> Self::Value {
         if scalar.is_bool() {
-            return self.trunc(val, self.cx().type_i1());
+            return self.unchecked_utrunc(val, self.cx().type_i1());
         }
         val
     }
diff --git a/compiler/rustc_codegen_gcc/src/intrinsic/mod.rs b/compiler/rustc_codegen_gcc/src/intrinsic/mod.rs
index a1123fafe2f..5322b731d8b 100644
--- a/compiler/rustc_codegen_gcc/src/intrinsic/mod.rs
+++ b/compiler/rustc_codegen_gcc/src/intrinsic/mod.rs
@@ -9,7 +9,7 @@ use gccjit::FunctionType;
 use gccjit::{ComparisonOp, Function, RValue, ToRValue, Type, UnaryOp};
 #[cfg(feature = "master")]
 use rustc_abi::ExternAbi;
-use rustc_abi::HasDataLayout;
+use rustc_abi::{BackendRepr, HasDataLayout};
 use rustc_codegen_ssa::MemFlags;
 use rustc_codegen_ssa::base::wants_msvc_seh;
 use rustc_codegen_ssa::common::IntPredicate;
@@ -181,14 +181,19 @@ impl<'a, 'gcc, 'tcx> IntrinsicCallBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tc
             sym::volatile_load | sym::unaligned_volatile_load => {
                 let tp_ty = fn_args.type_at(0);
                 let ptr = args[0].immediate();
+                let layout = self.layout_of(tp_ty);
                 let load = if let PassMode::Cast { cast: ref ty, pad_i32: _ } = fn_abi.ret.mode {
                     let gcc_ty = ty.gcc_type(self);
                     self.volatile_load(gcc_ty, ptr)
                 } else {
-                    self.volatile_load(self.layout_of(tp_ty).gcc_type(self), ptr)
+                    self.volatile_load(layout.gcc_type(self), ptr)
                 };
                 // TODO(antoyo): set alignment.
-                self.to_immediate(load, self.layout_of(tp_ty))
+                if let BackendRepr::Scalar(scalar) = layout.backend_repr {
+                    self.to_immediate_scalar(load, scalar)
+                } else {
+                    load
+                }
             }
             sym::volatile_store => {
                 let dst = args[0].deref(self.cx());