Rollup merge of rust-lang#103456 - scottmcm:fix-unchecked-shifts, r=scottmcm

`unchecked_{shl|shr}` should use `u32` as the RHS

The other shift methods, such as https://doc.rust-lang.org/nightly/std/primitive.u64.html#method.checked_shr and https://doc.rust-lang.org/nightly/std/primitive.i16.html#method.wrapping_shl, use `u32` for the shift amount.  That's consistent with other things, like `count_ones`, which also always use `u32` for a bit count, regardless of the size of the type.

This PR changes `unchecked_shl` and `unchecked_shr` to also use `u32` for the shift amount (rather than Self).

cc rust-lang#85122, the `unchecked_math` tracking issue

Author: Manishearth

library/core/src/lib.rs

@@ -131,6 +131,7 @@
 #![feature(const_pin)]
 #![feature(const_ptr_sub_ptr)]
 #![feature(const_replace)]
+#![feature(const_result_drop)]
 #![feature(const_ptr_as_ref)]
 #![feature(const_ptr_is_null)]
 #![feature(const_ptr_read)]

library/core/src/num/int_macros.rs

@@ -761,10 +761,11 @@ macro_rules! int_impl {
         #[rustc_const_unstable(feature = "const_inherent_unchecked_arith", issue = "85122")]
         #[inline(always)]
         #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
-        pub const unsafe fn unchecked_shl(self, rhs: Self) -> Self {
+        pub const unsafe fn unchecked_shl(self, rhs: u32) -> Self {
             // SAFETY: the caller must uphold the safety contract for
             // `unchecked_shl`.
-            unsafe { intrinsics::unchecked_shl(self, rhs) }
+            // Any legal shift amount is losslessly representable in the self type.
+            unsafe { intrinsics::unchecked_shl(self, rhs.try_into().ok().unwrap_unchecked()) }
         }
 
         /// Checked shift right. Computes `self >> rhs`, returning `None` if `rhs` is
@@ -808,10 +809,11 @@ macro_rules! int_impl {
         #[rustc_const_unstable(feature = "const_inherent_unchecked_arith", issue = "85122")]
         #[inline(always)]
         #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
-        pub const unsafe fn unchecked_shr(self, rhs: Self) -> Self {
+        pub const unsafe fn unchecked_shr(self, rhs: u32) -> Self {
             // SAFETY: the caller must uphold the safety contract for
             // `unchecked_shr`.
-            unsafe { intrinsics::unchecked_shr(self, rhs) }
+            // Any legal shift amount is losslessly representable in the self type.
+            unsafe { intrinsics::unchecked_shr(self, rhs.try_into().ok().unwrap_unchecked()) }
         }
 
         /// Checked absolute value. Computes `self.abs()`, returning `None` if
@@ -1358,11 +1360,12 @@ macro_rules! int_impl {
         #[must_use = "this returns the result of the operation, \
                       without modifying the original"]
         #[inline(always)]
+        #[rustc_allow_const_fn_unstable(const_inherent_unchecked_arith)]
         pub const fn wrapping_shl(self, rhs: u32) -> Self {
             // SAFETY: the masking by the bitsize of the type ensures that we do not shift
             // out of bounds
             unsafe {
-                intrinsics::unchecked_shl(self, (rhs & ($BITS - 1)) as $SelfT)
+                self.unchecked_shl(rhs & ($BITS - 1))
             }
         }
 
@@ -1387,11 +1390,12 @@ macro_rules! int_impl {
         #[must_use = "this returns the result of the operation, \
                       without modifying the original"]
         #[inline(always)]
+        #[rustc_allow_const_fn_unstable(const_inherent_unchecked_arith)]
         pub const fn wrapping_shr(self, rhs: u32) -> Self {
             // SAFETY: the masking by the bitsize of the type ensures that we do not shift
             // out of bounds
             unsafe {
-                intrinsics::unchecked_shr(self, (rhs & ($BITS - 1)) as $SelfT)
+                self.unchecked_shr(rhs & ($BITS - 1))
             }
         }
 

library/core/src/num/mod.rs

@@ -3,6 +3,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::ascii;
+use crate::convert::TryInto;
 use crate::error::Error;
 use crate::intrinsics;
 use crate::mem;

library/core/src/num/uint_macros.rs

@@ -908,10 +908,11 @@ macro_rules! uint_impl {
         #[rustc_const_unstable(feature = "const_inherent_unchecked_arith", issue = "85122")]
         #[inline(always)]
         #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
-        pub const unsafe fn unchecked_shl(self, rhs: Self) -> Self {
+        pub const unsafe fn unchecked_shl(self, rhs: u32) -> Self {
             // SAFETY: the caller must uphold the safety contract for
             // `unchecked_shl`.
-            unsafe { intrinsics::unchecked_shl(self, rhs) }
+            // Any legal shift amount is losslessly representable in the self type.
+            unsafe { intrinsics::unchecked_shl(self, rhs.try_into().ok().unwrap_unchecked()) }
         }
 
         /// Checked shift right. Computes `self >> rhs`, returning `None`
@@ -955,10 +956,11 @@ macro_rules! uint_impl {
         #[rustc_const_unstable(feature = "const_inherent_unchecked_arith", issue = "85122")]
         #[inline(always)]
         #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
-        pub const unsafe fn unchecked_shr(self, rhs: Self) -> Self {
+        pub const unsafe fn unchecked_shr(self, rhs: u32) -> Self {
             // SAFETY: the caller must uphold the safety contract for
             // `unchecked_shr`.
-            unsafe { intrinsics::unchecked_shr(self, rhs) }
+            // Any legal shift amount is losslessly representable in the self type.
+            unsafe { intrinsics::unchecked_shr(self, rhs.try_into().ok().unwrap_unchecked()) }
         }
 
         /// Checked exponentiation. Computes `self.pow(exp)`, returning `None` if
@@ -1374,11 +1376,12 @@ macro_rules! uint_impl {
         #[must_use = "this returns the result of the operation, \
                       without modifying the original"]
         #[inline(always)]
+        #[rustc_allow_const_fn_unstable(const_inherent_unchecked_arith)]
         pub const fn wrapping_shl(self, rhs: u32) -> Self {
             // SAFETY: the masking by the bitsize of the type ensures that we do not shift
             // out of bounds
             unsafe {
-                intrinsics::unchecked_shl(self, (rhs & ($BITS - 1)) as $SelfT)
+                self.unchecked_shl(rhs & ($BITS - 1))
             }
         }
 
@@ -1406,11 +1409,12 @@ macro_rules! uint_impl {
         #[must_use = "this returns the result of the operation, \
                       without modifying the original"]
         #[inline(always)]
+        #[rustc_allow_const_fn_unstable(const_inherent_unchecked_arith)]
         pub const fn wrapping_shr(self, rhs: u32) -> Self {
             // SAFETY: the masking by the bitsize of the type ensures that we do not shift
             // out of bounds
             unsafe {
-                intrinsics::unchecked_shr(self, (rhs & ($BITS - 1)) as $SelfT)
+                self.unchecked_shr(rhs & ($BITS - 1))
             }
         }
 

src/test/codegen/unchecked_shifts.rs

@@ -0,0 +1,66 @@
+// compile-flags: -O
+// min-llvm-version: 15.0 (LLVM 13 in CI does this differently from submodule LLVM)
+// ignore-debug (because unchecked is checked in debug)
+
+#![crate_type = "lib"]
+#![feature(unchecked_math)]
+
+// CHECK-LABEL: @unchecked_shl_unsigned_same
+#[no_mangle]
+pub unsafe fn unchecked_shl_unsigned_same(a: u32, b: u32) -> u32 {
+    // CHECK-NOT: and i32
+    // CHECK: shl i32 %a, %b
+    // CHECK-NOT: and i32
+    a.unchecked_shl(b)
+}
+
+// CHECK-LABEL: @unchecked_shl_unsigned_smaller
+#[no_mangle]
+pub unsafe fn unchecked_shl_unsigned_smaller(a: u16, b: u32) -> u16 {
+    // This uses -DAG to avoid failing on irrelevant reorderings,
+    // like emitting the truncation earlier.
+
+    // CHECK-DAG: %[[INRANGE:.+]] = icmp ult i32 %b, 65536
+    // CHECK-DAG: tail call void @llvm.assume(i1 %[[INRANGE]])
+    // CHECK-DAG: %[[TRUNC:.+]] = trunc i32 %b to i16
+    // CHECK-DAG: shl i16 %a, %[[TRUNC]]
+    a.unchecked_shl(b)
+}
+
+// CHECK-LABEL: @unchecked_shl_unsigned_bigger
+#[no_mangle]
+pub unsafe fn unchecked_shl_unsigned_bigger(a: u64, b: u32) -> u64 {
+    // CHECK: %[[EXT:.+]] = zext i32 %b to i64
+    // CHECK: shl i64 %a, %[[EXT]]
+    a.unchecked_shl(b)
+}
+
+// CHECK-LABEL: @unchecked_shr_signed_same
+#[no_mangle]
+pub unsafe fn unchecked_shr_signed_same(a: i32, b: u32) -> i32 {
+    // CHECK-NOT: and i32
+    // CHECK: ashr i32 %a, %b
+    // CHECK-NOT: and i32
+    a.unchecked_shr(b)
+}
+
+// CHECK-LABEL: @unchecked_shr_signed_smaller
+#[no_mangle]
+pub unsafe fn unchecked_shr_signed_smaller(a: i16, b: u32) -> i16 {
+    // This uses -DAG to avoid failing on irrelevant reorderings,
+    // like emitting the truncation earlier.
+
+    // CHECK-DAG: %[[INRANGE:.+]] = icmp ult i32 %b, 32768
+    // CHECK-DAG: tail call void @llvm.assume(i1 %[[INRANGE]])
+    // CHECK-DAG: %[[TRUNC:.+]] = trunc i32 %b to i16
+    // CHECK-DAG: ashr i16 %a, %[[TRUNC]]
+    a.unchecked_shr(b)
+}
+
+// CHECK-LABEL: @unchecked_shr_signed_bigger
+#[no_mangle]
+pub unsafe fn unchecked_shr_signed_bigger(a: i64, b: u32) -> i64 {
+    // CHECK: %[[EXT:.+]] = zext i32 %b to i64
+    // CHECK: ashr i64 %a, %[[EXT]]
+    a.unchecked_shr(b)
+}