Rollup merge of #143194 - folkertdev:fix-single-element-simd-bitcast, r=workingjubilee

fix bitcast of single-element SIMD vectors

in effect this reverts https://github.com/rust-lang/rust/pull/142768 and adds additional tests. That PR relaxed the conditions on an early return in an incorrect way that would create broken LLVM IR.

https://godbolt.org/z/PaaGWTv5a

```rust
#![feature(repr_simd)]

#[repr(simd)]
#[derive(Clone, Copy)]
struct S([i64; 1]);

#[no_mangle]
pub extern "C" fn single_element_simd(b: S) -> i64 {
    unsafe { std::mem::transmute(b) }
}
```
at the time of writing generates this LLVM IR, where the type of the return is different from the function's return type.

```llvm
define noundef i64 ``````@single_element_simd(<1`````` x i64> %b) unnamed_addr {
start:
  ret <1 x i64> %b
}
```

The test output is actually the same for the existing tests, showing that the change didn't actually matter for any tested behavior. It is probably a bit faster to do the early return, but, well, it's incorrect in general.

zullip thread: [#t-compiler > Is transmuting a &#96;T&#96; to &#96;Tx1&#96; (one-element SIMD vector) UB?](https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Is.20transmuting.20a.20.60T.60.20to.20.60Tx1.60.20.28one-element.20SIMD.20vector.29.20UB.3F/with/526262799)
cc ``````@sayantn``````
r? ``````@scottmcm``````

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -1117,7 +1117,7 @@ pub(super) fn transmute_immediate<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // While optimizations will remove no-op transmutes, they might still be
     // there in debug or things that aren't no-op in MIR because they change
     // the Rust type but not the underlying layout/niche.
-    if from_scalar == to_scalar {
+    if from_scalar == to_scalar && from_backend_ty == to_backend_ty {
         return imm;
     }
 
@@ -1136,13 +1136,7 @@ pub(super) fn transmute_immediate<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     assume_scalar_range(bx, imm, from_scalar, from_backend_ty);
 
     imm = match (from_scalar.primitive(), to_scalar.primitive()) {
-        (Int(..) | Float(_), Int(..) | Float(_)) => {
+        (Int(..) | Float(_), Int(..) | Float(_)) => bx.bitcast(imm, to_backend_ty),
-            if from_backend_ty == to_backend_ty {
-                imm
-            } else {
-                bx.bitcast(imm, to_backend_ty)
-            }
-        }
         (Pointer(..), Pointer(..)) => bx.pointercast(imm, to_backend_ty),
         (Int(..), Pointer(..)) => bx.ptradd(bx.const_null(bx.type_ptr()), imm),
         (Pointer(..), Int(..)) => {

tests/codegen/transmute-scalar.rs

@@ -1,6 +1,12 @@
+//@ add-core-stubs
 //@ compile-flags: -C opt-level=0 -C no-prepopulate-passes
 
 #![crate_type = "lib"]
+#![feature(no_core, repr_simd, arm_target_feature, mips_target_feature, s390x_target_feature)]
+#![no_core]
+extern crate minicore;
+
+use minicore::*;
 
 // With opaque ptrs in LLVM, `transmute` can load/store any `alloca` as any type,
 // without needing to pointercast, and SRoA will turn that into a `bitcast`.
@@ -14,7 +20,7 @@
 // CHECK-NEXT: ret i32 %_0
 #[no_mangle]
 pub fn f32_to_bits(x: f32) -> u32 {
-    unsafe { std::mem::transmute(x) }
+    unsafe { mem::transmute(x) }
 }
 
 // CHECK-LABEL: define{{.*}}i8 @bool_to_byte(i1 zeroext %b)
@@ -22,7 +28,7 @@ pub fn f32_to_bits(x: f32) -> u32 {
 // CHECK-NEXT: ret i8 %_0
 #[no_mangle]
 pub fn bool_to_byte(b: bool) -> u8 {
-    unsafe { std::mem::transmute(b) }
+    unsafe { mem::transmute(b) }
 }
 
 // CHECK-LABEL: define{{.*}}zeroext i1 @byte_to_bool(i8{{.*}} %byte)
@@ -30,14 +36,14 @@ pub fn bool_to_byte(b: bool) -> u8 {
 // CHECK-NEXT: ret i1 %_0
 #[no_mangle]
 pub unsafe fn byte_to_bool(byte: u8) -> bool {
-    std::mem::transmute(byte)
+    mem::transmute(byte)
 }
 
 // CHECK-LABEL: define{{.*}}ptr @ptr_to_ptr(ptr %p)
 // CHECK: ret ptr %p
 #[no_mangle]
 pub fn ptr_to_ptr(p: *mut u16) -> *mut u8 {
-    unsafe { std::mem::transmute(p) }
+    unsafe { mem::transmute(p) }
 }
 
 // CHECK: define{{.*}}[[USIZE:i[0-9]+]] @ptr_to_int(ptr %p)
@@ -45,7 +51,7 @@ pub fn ptr_to_ptr(p: *mut u16) -> *mut u8 {
 // CHECK-NEXT: ret [[USIZE]] %_0
 #[no_mangle]
 pub fn ptr_to_int(p: *mut u16) -> usize {
-    unsafe { std::mem::transmute(p) }
+    unsafe { mem::transmute(p) }
 }
 
 // CHECK: define{{.*}}ptr @int_to_ptr([[USIZE]] %i)
@@ -53,7 +59,7 @@ pub fn ptr_to_int(p: *mut u16) -> usize {
 // CHECK-NEXT: ret ptr %_0
 #[no_mangle]
 pub fn int_to_ptr(i: usize) -> *mut u16 {
-    unsafe { std::mem::transmute(i) }
+    unsafe { mem::transmute(i) }
 }
 
 // This is the one case where signedness matters to transmuting:
@@ -70,7 +76,7 @@ pub enum FakeBoolSigned {
 // CHECK-NEXT: ret i8 %_0
 #[no_mangle]
 pub fn bool_to_fake_bool_signed(b: bool) -> FakeBoolSigned {
-    unsafe { std::mem::transmute(b) }
+    unsafe { mem::transmute(b) }
 }
 
 // CHECK-LABEL: define{{.*}}i1 @fake_bool_signed_to_bool(i8 %b)
@@ -78,7 +84,7 @@ pub fn bool_to_fake_bool_signed(b: bool) -> FakeBoolSigned {
 // CHECK-NEXT: ret i1 %_0
 #[no_mangle]
 pub fn fake_bool_signed_to_bool(b: FakeBoolSigned) -> bool {
-    unsafe { std::mem::transmute(b) }
+    unsafe { mem::transmute(b) }
 }
 
 #[repr(u8)]
@@ -91,12 +97,41 @@ pub enum FakeBoolUnsigned {
 // CHECK: ret i1 %b
 #[no_mangle]
 pub fn bool_to_fake_bool_unsigned(b: bool) -> FakeBoolUnsigned {
-    unsafe { std::mem::transmute(b) }
+    unsafe { mem::transmute(b) }
 }
 
 // CHECK-LABEL: define{{.*}}i1 @fake_bool_unsigned_to_bool(i1 zeroext %b)
 // CHECK: ret i1 %b
 #[no_mangle]
 pub fn fake_bool_unsigned_to_bool(b: FakeBoolUnsigned) -> bool {
-    unsafe { std::mem::transmute(b) }
+    unsafe { mem::transmute(b) }
+}
+
+#[repr(simd)]
+struct S([i64; 1]);
+
+// CHECK-LABEL: define{{.*}}i64 @single_element_simd_to_scalar(<1 x i64> %b)
+// CHECK: bitcast <1 x i64> %b to i64
+// CHECK: ret i64
+#[no_mangle]
+#[cfg_attr(target_family = "wasm", target_feature(enable = "simd128"))]
+#[cfg_attr(target_arch = "arm", target_feature(enable = "neon"))]
+#[cfg_attr(target_arch = "x86", target_feature(enable = "sse"))]
+#[cfg_attr(target_arch = "mips", target_feature(enable = "msa"))]
+#[cfg_attr(target_arch = "s390x", target_feature(enable = "vector"))]
+pub extern "C" fn single_element_simd_to_scalar(b: S) -> i64 {
+    unsafe { mem::transmute(b) }
+}
+
+// CHECK-LABEL: define{{.*}}<1 x i64> @scalar_to_single_element_simd(i64 %b)
+// CHECK: bitcast i64 %b to <1 x i64>
+// CHECK: ret <1 x i64>
+#[no_mangle]
+#[cfg_attr(target_family = "wasm", target_feature(enable = "simd128"))]
+#[cfg_attr(target_arch = "arm", target_feature(enable = "neon"))]
+#[cfg_attr(target_arch = "x86", target_feature(enable = "sse"))]
+#[cfg_attr(target_arch = "mips", target_feature(enable = "msa"))]
+#[cfg_attr(target_arch = "s390x", target_feature(enable = "vector"))]
+pub extern "C" fn scalar_to_single_element_simd(b: i64) -> S {
+    unsafe { mem::transmute(b) }
 }