Rollup merge of #109475 - scottmcm:simpler-shifts, r=WaffleLapkin

Simpler checked shifts in MIR building

Doing masking to check unsigned shift amounts is overcomplicated; just comparing the shift directly saves a statement and a temporary, as well as is much easier to read as a human.  And shifting by unsigned is the canonical case -- notably, all the library shifting methods (that don't support every type) take shift RHSs as `u32` -- so we might as well make that simpler since it's easy to do so.

This PR also changes *signed* shift amounts to `IntToInt` casts and then uses the same check as for unsigned.  The bit-masking is a nice trick, but for example LLVM actually canonicalizes it to an unsigned comparison anyway <https://rust.godbolt.org/z/8h59fMGT4> so I don't think it's worth the effort and the extra `Constant`.  (If MIR's `assert` was `assert_nz` then the masking might make sense, but when the `!=` uses another statement I think the comparison is better.)

To review, I suggest looking at 2ee0468 first -- that's the interesting code change and has a MIR diff.

My favourite part of the diff:
```diff
-        _20 = BitAnd(_19, const 340282366920938463463374607431768211448_u128); // scope 0 at $DIR/shifts.rs:+2:34: +2:44
-        _21 = Ne(move _20, const 0_u128); // scope 0 at $DIR/shifts.rs:+2:34: +2:44
-        assert(!move _21, "attempt to shift right by `{}`, which would overflow", _19) -> [success: bb3, unwind: bb7]; // scope 0 at $DIR/shifts.rs:+2:34: +2:44
+        _18 = Lt(_17, const 8_u128);     // scope 0 at $DIR/shifts.rs:+2:34: +2:44
+        assert(move _18, "attempt to shift right by `{}`, which would overflow", _17) -> [success: bb3, unwind: bb7]; // scope 0 at $DIR/shifts.rs:+2:34: +2:44
```

Author: matthiaskrgr

compiler/rustc_mir_build/src/build/expr/as_rvalue.rs

@@ -566,41 +566,51 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 Rvalue::Use(Operand::Move(val))
             }
             BinOp::Shl | BinOp::Shr if self.check_overflow && ty.is_integral() => {
-                // Consider that the shift overflows if `rhs < 0` or `rhs >= bits`.
-                // This can be encoded as a single operation as `(rhs & -bits) != 0`.
-                let (size, _) = ty.int_size_and_signed(self.tcx);
-                let bits = size.bits();
-                debug_assert!(bits.is_power_of_two());
-                let mask = !((bits - 1) as u128);
-
+                // For an unsigned RHS, the shift is in-range for `rhs < bits`.
+                // For a signed RHS, `IntToInt` cast to the equivalent unsigned
+                // type and do that same comparison.  Because the type is the
+                // same size, there's no negative shift amount that ends up
+                // overlapping with valid ones, thus it catches negatives too.
+                let (lhs_size, _) = ty.int_size_and_signed(self.tcx);
                 let rhs_ty = rhs.ty(&self.local_decls, self.tcx);
                 let (rhs_size, _) = rhs_ty.int_size_and_signed(self.tcx);
-                let mask = Operand::const_from_scalar(
+
+                let (unsigned_rhs, unsigned_ty) = match rhs_ty.kind() {
+                    ty::Uint(_) => (rhs.to_copy(), rhs_ty),
+                    ty::Int(int_width) => {
+                        let uint_ty = self.tcx.mk_mach_uint(int_width.to_unsigned());
+                        let rhs_temp = self.temp(uint_ty, span);
+                        self.cfg.push_assign(
+                            block,
+                            source_info,
+                            rhs_temp,
+                            Rvalue::Cast(CastKind::IntToInt, rhs.to_copy(), uint_ty),
+                        );
+                        (Operand::Move(rhs_temp), uint_ty)
+                    }
+                    _ => unreachable!("only integers are shiftable"),
+                };
+
+                // This can't overflow because the largest shiftable types are 128-bit,
+                // which fits in `u8`, the smallest possible `unsigned_ty`.
+                // (And `from_uint` will `bug!` if that's ever no longer true.)
+                let lhs_bits = Operand::const_from_scalar(
                     self.tcx,
-                    rhs_ty,
-                    Scalar::from_uint(rhs_size.truncate(mask), rhs_size),
+                    unsigned_ty,
+                    Scalar::from_uint(lhs_size.bits(), rhs_size),
                     span,
                 );
 
-                let outer_bits = self.temp(rhs_ty, span);
-                self.cfg.push_assign(
-                    block,
-                    source_info,
-                    outer_bits,
-                    Rvalue::BinaryOp(BinOp::BitAnd, Box::new((rhs.to_copy(), mask))),
-                );
-
-                let overflows = self.temp(bool_ty, span);
-                let zero = self.zero_literal(span, rhs_ty);
+                let inbounds = self.temp(bool_ty, span);
                 self.cfg.push_assign(
                     block,
                     source_info,
-                    overflows,
-                    Rvalue::BinaryOp(BinOp::Ne, Box::new((Operand::Move(outer_bits), zero))),
+                    inbounds,
+                    Rvalue::BinaryOp(BinOp::Lt, Box::new((unsigned_rhs, lhs_bits))),
                 );
 
                 let overflow_err = AssertKind::Overflow(op, lhs.to_copy(), rhs.to_copy());
-                block = self.assert(block, Operand::Move(overflows), false, overflow_err, span);
+                block = self.assert(block, Operand::Move(inbounds), true, overflow_err, span);
                 Rvalue::BinaryOp(op, Box::new((lhs, rhs)))
             }
             BinOp::Div | BinOp::Rem if ty.is_integral() => {

compiler/rustc_type_ir/src/lib.rs

@@ -432,6 +432,17 @@ impl IntTy {
             _ => *self,
         }
     }
+
+    pub fn to_unsigned(self) -> UintTy {
+        match self {
+            IntTy::Isize => UintTy::Usize,
+            IntTy::I8 => UintTy::U8,
+            IntTy::I16 => UintTy::U16,
+            IntTy::I32 => UintTy::U32,
+            IntTy::I64 => UintTy::U64,
+            IntTy::I128 => UintTy::U128,
+        }
+    }
 }
 
 #[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Copy, Debug)]
@@ -479,6 +490,17 @@ impl UintTy {
             _ => *self,
         }
     }
+
+    pub fn to_signed(self) -> IntTy {
+        match self {
+            UintTy::Usize => IntTy::Isize,
+            UintTy::U8 => IntTy::I8,
+            UintTy::U16 => IntTy::I16,
+            UintTy::U32 => IntTy::I32,
+            UintTy::U64 => IntTy::I64,
+            UintTy::U128 => IntTy::I128,
+        }
+    }
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]

tests/mir-opt/building/shifts.rs

@@ -0,0 +1,20 @@
+// compile-flags: -C debug-assertions=yes
+
+// EMIT_MIR shifts.shift_signed.built.after.mir
+fn shift_signed(small: i8, big: u128, a: i8, b: i32, c: i128) -> ([i8; 3], [u128; 3]) {
+    (
+        [small >> a, small >> b, small >> c],
+        [big << a, big << b, big << c],
+    )
+}
+
+// EMIT_MIR shifts.shift_unsigned.built.after.mir
+fn shift_unsigned(small: u8, big: i128, a: u8, b: u32, c: u128) -> ([u8; 3], [i128; 3]) {
+    (
+        [small >> a, small >> b, small >> c],
+        [big << a, big << b, big << c],
+    )
+}
+
+fn main() {
+}

tests/mir-opt/building/shifts.shift_signed.built.after.mir

@@ -0,0 +1,147 @@
+// MIR for `shift_signed` after built
+
+fn shift_signed(_1: i8, _2: u128, _3: i8, _4: i32, _5: i128) -> ([i8; 3], [u128; 3]) {
+    debug small => _1;                   // in scope 0 at $DIR/shifts.rs:+0:17: +0:22
+    debug big => _2;                     // in scope 0 at $DIR/shifts.rs:+0:28: +0:31
+    debug a => _3;                       // in scope 0 at $DIR/shifts.rs:+0:39: +0:40
+    debug b => _4;                       // in scope 0 at $DIR/shifts.rs:+0:46: +0:47
+    debug c => _5;                       // in scope 0 at $DIR/shifts.rs:+0:54: +0:55
+    let mut _0: ([i8; 3], [u128; 3]);    // return place in scope 0 at $DIR/shifts.rs:+0:66: +0:86
+    let mut _6: [i8; 3];                 // in scope 0 at $DIR/shifts.rs:+2:9: +2:45
+    let mut _7: i8;                      // in scope 0 at $DIR/shifts.rs:+2:10: +2:20
+    let mut _8: i8;                      // in scope 0 at $DIR/shifts.rs:+2:10: +2:15
+    let mut _9: i8;                      // in scope 0 at $DIR/shifts.rs:+2:19: +2:20
+    let mut _10: u8;                     // in scope 0 at $DIR/shifts.rs:+2:10: +2:20
+    let mut _11: bool;                   // in scope 0 at $DIR/shifts.rs:+2:10: +2:20
+    let mut _12: i8;                     // in scope 0 at $DIR/shifts.rs:+2:22: +2:32
+    let mut _13: i8;                     // in scope 0 at $DIR/shifts.rs:+2:22: +2:27
+    let mut _14: i32;                    // in scope 0 at $DIR/shifts.rs:+2:31: +2:32
+    let mut _15: u32;                    // in scope 0 at $DIR/shifts.rs:+2:22: +2:32
+    let mut _16: bool;                   // in scope 0 at $DIR/shifts.rs:+2:22: +2:32
+    let mut _17: i8;                     // in scope 0 at $DIR/shifts.rs:+2:34: +2:44
+    let mut _18: i8;                     // in scope 0 at $DIR/shifts.rs:+2:34: +2:39
+    let mut _19: i128;                   // in scope 0 at $DIR/shifts.rs:+2:43: +2:44
+    let mut _20: u128;                   // in scope 0 at $DIR/shifts.rs:+2:34: +2:44
+    let mut _21: bool;                   // in scope 0 at $DIR/shifts.rs:+2:34: +2:44
+    let mut _22: [u128; 3];              // in scope 0 at $DIR/shifts.rs:+3:9: +3:39
+    let mut _23: u128;                   // in scope 0 at $DIR/shifts.rs:+3:10: +3:18
+    let mut _24: u128;                   // in scope 0 at $DIR/shifts.rs:+3:10: +3:13
+    let mut _25: i8;                     // in scope 0 at $DIR/shifts.rs:+3:17: +3:18
+    let mut _26: u8;                     // in scope 0 at $DIR/shifts.rs:+3:10: +3:18
+    let mut _27: bool;                   // in scope 0 at $DIR/shifts.rs:+3:10: +3:18
+    let mut _28: u128;                   // in scope 0 at $DIR/shifts.rs:+3:20: +3:28
+    let mut _29: u128;                   // in scope 0 at $DIR/shifts.rs:+3:20: +3:23
+    let mut _30: i32;                    // in scope 0 at $DIR/shifts.rs:+3:27: +3:28
+    let mut _31: u32;                    // in scope 0 at $DIR/shifts.rs:+3:20: +3:28
+    let mut _32: bool;                   // in scope 0 at $DIR/shifts.rs:+3:20: +3:28
+    let mut _33: u128;                   // in scope 0 at $DIR/shifts.rs:+3:30: +3:38
+    let mut _34: u128;                   // in scope 0 at $DIR/shifts.rs:+3:30: +3:33
+    let mut _35: i128;                   // in scope 0 at $DIR/shifts.rs:+3:37: +3:38
+    let mut _36: u128;                   // in scope 0 at $DIR/shifts.rs:+3:30: +3:38
+    let mut _37: bool;                   // in scope 0 at $DIR/shifts.rs:+3:30: +3:38
+
+    bb0: {
+        StorageLive(_6);                 // scope 0 at $DIR/shifts.rs:+2:9: +2:45
+        StorageLive(_7);                 // scope 0 at $DIR/shifts.rs:+2:10: +2:20
+        StorageLive(_8);                 // scope 0 at $DIR/shifts.rs:+2:10: +2:15
+        _8 = _1;                         // scope 0 at $DIR/shifts.rs:+2:10: +2:15
+        StorageLive(_9);                 // scope 0 at $DIR/shifts.rs:+2:19: +2:20
+        _9 = _3;                         // scope 0 at $DIR/shifts.rs:+2:19: +2:20
+        _10 = _9 as u8 (IntToInt);       // scope 0 at $DIR/shifts.rs:+2:10: +2:20
+        _11 = Lt(move _10, const 8_u8);  // scope 0 at $DIR/shifts.rs:+2:10: +2:20
+        assert(move _11, "attempt to shift right by `{}`, which would overflow", _9) -> [success: bb1, unwind: bb7]; // scope 0 at $DIR/shifts.rs:+2:10: +2:20
+    }
+
+    bb1: {
+        _7 = Shr(move _8, move _9);      // scope 0 at $DIR/shifts.rs:+2:10: +2:20
+        StorageDead(_9);                 // scope 0 at $DIR/shifts.rs:+2:19: +2:20
+        StorageDead(_8);                 // scope 0 at $DIR/shifts.rs:+2:19: +2:20
+        StorageLive(_12);                // scope 0 at $DIR/shifts.rs:+2:22: +2:32
+        StorageLive(_13);                // scope 0 at $DIR/shifts.rs:+2:22: +2:27
+        _13 = _1;                        // scope 0 at $DIR/shifts.rs:+2:22: +2:27
+        StorageLive(_14);                // scope 0 at $DIR/shifts.rs:+2:31: +2:32
+        _14 = _4;                        // scope 0 at $DIR/shifts.rs:+2:31: +2:32
+        _15 = _14 as u32 (IntToInt);     // scope 0 at $DIR/shifts.rs:+2:22: +2:32
+        _16 = Lt(move _15, const 8_u32); // scope 0 at $DIR/shifts.rs:+2:22: +2:32
+        assert(move _16, "attempt to shift right by `{}`, which would overflow", _14) -> [success: bb2, unwind: bb7]; // scope 0 at $DIR/shifts.rs:+2:22: +2:32
+    }
+
+    bb2: {
+        _12 = Shr(move _13, move _14);   // scope 0 at $DIR/shifts.rs:+2:22: +2:32
+        StorageDead(_14);                // scope 0 at $DIR/shifts.rs:+2:31: +2:32
+        StorageDead(_13);                // scope 0 at $DIR/shifts.rs:+2:31: +2:32
+        StorageLive(_17);                // scope 0 at $DIR/shifts.rs:+2:34: +2:44
+        StorageLive(_18);                // scope 0 at $DIR/shifts.rs:+2:34: +2:39
+        _18 = _1;                        // scope 0 at $DIR/shifts.rs:+2:34: +2:39
+        StorageLive(_19);                // scope 0 at $DIR/shifts.rs:+2:43: +2:44
+        _19 = _5;                        // scope 0 at $DIR/shifts.rs:+2:43: +2:44
+        _20 = _19 as u128 (IntToInt);    // scope 0 at $DIR/shifts.rs:+2:34: +2:44
+        _21 = Lt(move _20, const 8_u128); // scope 0 at $DIR/shifts.rs:+2:34: +2:44
+        assert(move _21, "attempt to shift right by `{}`, which would overflow", _19) -> [success: bb3, unwind: bb7]; // scope 0 at $DIR/shifts.rs:+2:34: +2:44
+    }
+
+    bb3: {
+        _17 = Shr(move _18, move _19);   // scope 0 at $DIR/shifts.rs:+2:34: +2:44
+        StorageDead(_19);                // scope 0 at $DIR/shifts.rs:+2:43: +2:44
+        StorageDead(_18);                // scope 0 at $DIR/shifts.rs:+2:43: +2:44
+        _6 = [move _7, move _12, move _17]; // scope 0 at $DIR/shifts.rs:+2:9: +2:45
+        StorageDead(_17);                // scope 0 at $DIR/shifts.rs:+2:44: +2:45
+        StorageDead(_12);                // scope 0 at $DIR/shifts.rs:+2:44: +2:45
+        StorageDead(_7);                 // scope 0 at $DIR/shifts.rs:+2:44: +2:45
+        StorageLive(_22);                // scope 0 at $DIR/shifts.rs:+3:9: +3:39
+        StorageLive(_23);                // scope 0 at $DIR/shifts.rs:+3:10: +3:18
+        StorageLive(_24);                // scope 0 at $DIR/shifts.rs:+3:10: +3:13
+        _24 = _2;                        // scope 0 at $DIR/shifts.rs:+3:10: +3:13
+        StorageLive(_25);                // scope 0 at $DIR/shifts.rs:+3:17: +3:18
+        _25 = _3;                        // scope 0 at $DIR/shifts.rs:+3:17: +3:18
+        _26 = _25 as u8 (IntToInt);      // scope 0 at $DIR/shifts.rs:+3:10: +3:18
+        _27 = Lt(move _26, const 128_u8); // scope 0 at $DIR/shifts.rs:+3:10: +3:18
+        assert(move _27, "attempt to shift left by `{}`, which would overflow", _25) -> [success: bb4, unwind: bb7]; // scope 0 at $DIR/shifts.rs:+3:10: +3:18
+    }
+
+    bb4: {
+        _23 = Shl(move _24, move _25);   // scope 0 at $DIR/shifts.rs:+3:10: +3:18
+        StorageDead(_25);                // scope 0 at $DIR/shifts.rs:+3:17: +3:18
+        StorageDead(_24);                // scope 0 at $DIR/shifts.rs:+3:17: +3:18
+        StorageLive(_28);                // scope 0 at $DIR/shifts.rs:+3:20: +3:28
+        StorageLive(_29);                // scope 0 at $DIR/shifts.rs:+3:20: +3:23
+        _29 = _2;                        // scope 0 at $DIR/shifts.rs:+3:20: +3:23
+        StorageLive(_30);                // scope 0 at $DIR/shifts.rs:+3:27: +3:28
+        _30 = _4;                        // scope 0 at $DIR/shifts.rs:+3:27: +3:28
+        _31 = _30 as u32 (IntToInt);     // scope 0 at $DIR/shifts.rs:+3:20: +3:28
+        _32 = Lt(move _31, const 128_u32); // scope 0 at $DIR/shifts.rs:+3:20: +3:28
+        assert(move _32, "attempt to shift left by `{}`, which would overflow", _30) -> [success: bb5, unwind: bb7]; // scope 0 at $DIR/shifts.rs:+3:20: +3:28
+    }
+
+    bb5: {
+        _28 = Shl(move _29, move _30);   // scope 0 at $DIR/shifts.rs:+3:20: +3:28
+        StorageDead(_30);                // scope 0 at $DIR/shifts.rs:+3:27: +3:28
+        StorageDead(_29);                // scope 0 at $DIR/shifts.rs:+3:27: +3:28
+        StorageLive(_33);                // scope 0 at $DIR/shifts.rs:+3:30: +3:38
+        StorageLive(_34);                // scope 0 at $DIR/shifts.rs:+3:30: +3:33
+        _34 = _2;                        // scope 0 at $DIR/shifts.rs:+3:30: +3:33
+        StorageLive(_35);                // scope 0 at $DIR/shifts.rs:+3:37: +3:38
+        _35 = _5;                        // scope 0 at $DIR/shifts.rs:+3:37: +3:38
+        _36 = _35 as u128 (IntToInt);    // scope 0 at $DIR/shifts.rs:+3:30: +3:38
+        _37 = Lt(move _36, const 128_u128); // scope 0 at $DIR/shifts.rs:+3:30: +3:38
+        assert(move _37, "attempt to shift left by `{}`, which would overflow", _35) -> [success: bb6, unwind: bb7]; // scope 0 at $DIR/shifts.rs:+3:30: +3:38
+    }
+
+    bb6: {
+        _33 = Shl(move _34, move _35);   // scope 0 at $DIR/shifts.rs:+3:30: +3:38
+        StorageDead(_35);                // scope 0 at $DIR/shifts.rs:+3:37: +3:38
+        StorageDead(_34);                // scope 0 at $DIR/shifts.rs:+3:37: +3:38
+        _22 = [move _23, move _28, move _33]; // scope 0 at $DIR/shifts.rs:+3:9: +3:39
+        StorageDead(_33);                // scope 0 at $DIR/shifts.rs:+3:38: +3:39
+        StorageDead(_28);                // scope 0 at $DIR/shifts.rs:+3:38: +3:39
+        StorageDead(_23);                // scope 0 at $DIR/shifts.rs:+3:38: +3:39
+        _0 = (move _6, move _22);        // scope 0 at $DIR/shifts.rs:+1:5: +4:6
+        StorageDead(_22);                // scope 0 at $DIR/shifts.rs:+4:5: +4:6
+        StorageDead(_6);                 // scope 0 at $DIR/shifts.rs:+4:5: +4:6
+        return;                          // scope 0 at $DIR/shifts.rs:+5:2: +5:2
+    }
+
+    bb7 (cleanup): {
+        resume;                          // scope 0 at $DIR/shifts.rs:+0:1: +5:2
+    }
+}