Add transmute optimization tests and some extra comments

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -290,7 +290,13 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
 
         use abi::Primitive::*;
         imm = bx.from_immediate(imm);
+
+        // When scalars are passed by value, there's no metadata recording their
+        // valid ranges. For example, `char`s are passed as just `i32`, with no
+        // way for LLVM to know that they're 0x10FFFF at most. Thus we assume
+        // the range of the input value too, not just the output range.
         self.assume_scalar_range(bx, imm, from_scalar, from_backend_ty);
+
         imm = match (from_scalar.primitive(), to_scalar.primitive()) {
             (Int(..) | F32 | F64, Int(..) | F32 | F64) => bx.bitcast(imm, to_backend_ty),
             (Pointer(..), Pointer(..)) => bx.pointercast(imm, to_backend_ty),
@@ -318,6 +324,9 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         backend_ty: Bx::Type,
     ) {
         if matches!(self.cx.sess().opts.optimize, OptLevel::No | OptLevel::Less)
+            // For now, the critical niches are all over `Int`eger values.
+            // Should floating-point values or pointers ever get more complex
+            // niches, then this code will probably want to handle them too.
             || !matches!(scalar.primitive(), abi::Primitive::Int(..))
             || scalar.is_always_valid(self.cx)
         {

tests/codegen/transmute-optimized.rs

@@ -0,0 +1,109 @@
+// compile-flags: -O -Z merge-functions=disabled
+// min-llvm-version: 15.0 # this test uses `ptr`s
+// ignore-debug
+
+#![crate_type = "lib"]
+
+// This tests that LLVM can optimize based on the niches in the source or
+// destination types for transmutes.
+
+#[repr(u32)]
+pub enum AlwaysZero32 { X = 0 }
+
+// CHECK-LABEL: i32 @issue_109958(i32
+#[no_mangle]
+pub fn issue_109958(x: AlwaysZero32) -> i32 {
+    // CHECK: ret i32 0
+    unsafe { std::mem::transmute(x) }
+}
+
+// CHECK-LABEL: i1 @reference_is_null(ptr
+#[no_mangle]
+pub fn reference_is_null(x: &i32) -> bool {
+    // CHECK: ret i1 false
+    let p: *const i32 = unsafe { std::mem::transmute(x) };
+    p.is_null()
+}
+
+// CHECK-LABEL: i1 @non_null_is_null(ptr
+#[no_mangle]
+pub fn non_null_is_null(x: std::ptr::NonNull<i32>) -> bool {
+    // CHECK: ret i1 false
+    let p: *const i32 = unsafe { std::mem::transmute(x) };
+    p.is_null()
+}
+
+// CHECK-LABEL: i1 @non_zero_is_null(
+#[no_mangle]
+pub fn non_zero_is_null(x: std::num::NonZeroUsize) -> bool {
+    // CHECK: ret i1 false
+    let p: *const i32 = unsafe { std::mem::transmute(x) };
+    p.is_null()
+}
+
+// CHECK-LABEL: i1 @non_null_is_zero(ptr
+#[no_mangle]
+pub fn non_null_is_zero(x: std::ptr::NonNull<i32>) -> bool {
+    // CHECK: ret i1 false
+    let a: isize = unsafe { std::mem::transmute(x) };
+    a == 0
+}
+
+// CHECK-LABEL: i1 @bool_ordering_is_ge(i1
+#[no_mangle]
+pub fn bool_ordering_is_ge(x: bool) -> bool {
+    // CHECK: ret i1 true
+    let y: std::cmp::Ordering = unsafe { std::mem::transmute(x) };
+    y.is_ge()
+}
+
+// CHECK-LABEL: i1 @ordering_is_ge_then_transmute_to_bool(i8
+#[no_mangle]
+pub fn ordering_is_ge_then_transmute_to_bool(x: std::cmp::Ordering) -> bool {
+    let r = x.is_ge();
+    let _: bool = unsafe { std::mem::transmute(x) };
+    r
+}
+
+// CHECK-LABEL: i32 @normal_div(i32
+#[no_mangle]
+pub fn normal_div(a: u32, b: u32) -> u32 {
+    // CHECK: call core::panicking::panic
+    a / b
+}
+
+// CHECK-LABEL: i32 @div_transmute_nonzero(i32
+#[no_mangle]
+pub fn div_transmute_nonzero(a: u32, b: std::num::NonZeroI32) -> u32 {
+    // CHECK-NOT: call core::panicking::panic
+    // CHECK: %[[R:.+]] = udiv i32 %a, %b
+    // CHECK-NEXT: ret i32 %[[R]]
+    // CHECK-NOT: call core::panicking::panic
+    let d: u32 = unsafe { std::mem::transmute(b) };
+    a / d
+}
+
+#[repr(i8)]
+pub enum OneTwoThree { One = 1, Two = 2, Three = 3 }
+
+// CHECK-LABEL: i8 @ordering_transmute_onetwothree(i8
+#[no_mangle]
+pub unsafe fn ordering_transmute_onetwothree(x: std::cmp::Ordering) -> OneTwoThree {
+    // CHECK: ret i8 1
+    std::mem::transmute(x)
+}
+
+// CHECK-LABEL: i8 @onetwothree_transmute_ordering(i8
+#[no_mangle]
+pub unsafe fn onetwothree_transmute_ordering(x: OneTwoThree) -> std::cmp::Ordering {
+    // CHECK: ret i8 1
+    std::mem::transmute(x)
+}
+
+// CHECK-LABEL: i1 @char_is_negative(i32
+#[no_mangle]
+pub fn char_is_negative(c: char) -> bool {
+    // CHECK: ret i1 false
+    let x: i32 = unsafe { std::mem::transmute(c) };
+    x < 0
+}