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interpret: simplify handling of shifts by no longer trying to handle signed and unsigned shift amounts in the same branch

This commit is contained in:
Ralf Jung 2023-11-12 12:16:41 +01:00
parent a04d56b36d
commit 31493c70fa
7 changed files with 86 additions and 58 deletions
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9
compiler/rustc_codegen_ssa/src/base.rs
View File

@ -322,8 +322,13 @@ pub fn cast_shift_expr_rhs<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
if lhs_sz < rhs_sz {
bx.trunc(rhs, lhs_llty)
} else if lhs_sz > rhs_sz {
// FIXME (#1877: If in the future shifting by negative
// values is no longer undefined then this is wrong.
// We zero-extend even if the RHS is signed. So e.g. `(x: i32) << -1i8` will zero-extend the
// RHS to `255i32`. But then we mask the shift amount to be within the size of the LHS
// anyway so the result is `31` as it should be. All the extra bits introduced by zext
// are masked off so their value does not matter.
// FIXME: if we ever support 512bit integers, this will be wrong! For such large integers,
// the extra bits introduced by zext are *not* all masked away any more.
assert!(lhs_sz <= 256);
bx.zext(rhs, lhs_llty)
} else {
rhs

54
compiler/rustc_const_eval/src/interpret/operator.rs
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@ -157,41 +157,35 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
// Shift ops can have an RHS with a different numeric type.
if matches!(bin_op, Shl | ShlUnchecked | Shr | ShrUnchecked) {
let size = u128::from(left_layout.size.bits());
// Even if `r` is signed, we treat it as if it was unsigned (i.e., we use its
// zero-extended form). This matches the codegen backend:
// <https://github.com/rust-lang/rust/blob/c274e4969f058b1c644243181ece9f829efa7594/compiler/rustc_codegen_ssa/src/base.rs#L315-L317>.
// The overflow check is also ignorant to the sign:
// <https://github.com/rust-lang/rust/blob/c274e4969f058b1c644243181ece9f829efa7594/compiler/rustc_codegen_ssa/src/mir/rvalue.rs#L728>.
// This would behave rather strangely if we had integer types of size 256: a shift by
// -1i8 would actually shift by 255, but that would *not* be considered overflowing. A
// shift by -1i16 though would be considered overflowing. If we had integers of size
// 512, then a shift by -1i8 would even produce a different result than one by -1i16:
// the first shifts by 255, the latter by u16::MAX % 512 = 511. Lucky enough, our
// integers are maximally 128bits wide, so negative shifts *always* overflow and we have
// consistent results for the same value represented at different bit widths.
assert!(size <= 128);
let original_r = r;
let overflow = r >= size;
// The shift offset is implicitly masked to the type size, to make sure this operation
// is always defined. This is the one MIR operator that does *not* directly map to a
// single LLVM operation. See
// <https://github.com/rust-lang/rust/blob/c274e4969f058b1c644243181ece9f829efa7594/compiler/rustc_codegen_ssa/src/common.rs#L131-L158>
// for the corresponding truncation in our codegen backends.
let r = r % size;
let r = u32::try_from(r).unwrap(); // we masked so this will always fit
let size = left_layout.size.bits();
// The shift offset is implicitly masked to the type size. (This is the one MIR operator
// that does *not* directly map to a single LLVM operation.) Compute how much we
// actually shift and whether there was an overflow due to shifting too much.
let (shift_amount, overflow) = if right_layout.abi.is_signed() {
let shift_amount = self.sign_extend(r, right_layout) as i128;
let overflow = shift_amount < 0 || shift_amount >= i128::from(size);
let masked_amount = (shift_amount as u128) % u128::from(size);
debug_assert_eq!(overflow, shift_amount != (masked_amount as i128));
(masked_amount, overflow)
} else {
let shift_amount = r;
let masked_amount = shift_amount % u128::from(size);
(masked_amount, shift_amount != masked_amount)
};
let shift_amount = u32::try_from(shift_amount).unwrap(); // we masked so this will always fit
// Compute the shifted result.
let result = if left_layout.abi.is_signed() {
let l = self.sign_extend(l, left_layout) as i128;
let result = match bin_op {
Shl | ShlUnchecked => l.checked_shl(r).unwrap(),
Shr | ShrUnchecked => l.checked_shr(r).unwrap(),
Shl | ShlUnchecked => l.checked_shl(shift_amount).unwrap(),
Shr | ShrUnchecked => l.checked_shr(shift_amount).unwrap(),
_ => bug!(),
};
result as u128
} else {
match bin_op {
Shl | ShlUnchecked => l.checked_shl(r).unwrap(),
Shr | ShrUnchecked => l.checked_shr(r).unwrap(),
Shl | ShlUnchecked => l.checked_shl(shift_amount).unwrap(),
Shr | ShrUnchecked => l.checked_shr(shift_amount).unwrap(),
_ => bug!(),
}
};
@ -200,7 +194,11 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
if overflow && let Some(intrinsic_name) = throw_ub_on_overflow {
throw_ub_custom!(
fluent::const_eval_overflow_shift,
val = original_r,
val = if right_layout.abi.is_signed() {
(self.sign_extend(r, right_layout) as i128).to_string()
} else {
r.to_string()
},
name = intrinsic_name
);
}

8
compiler/rustc_middle/src/mir/syntax.rs
View File

@ -1404,18 +1404,18 @@ pub enum BinOp {
BitOr,
/// The `<<` operator (shift left)
///
/// The offset is truncated to the size of the first operand before shifting.
/// The offset is truncated to the size of the first operand and made unsigned before shifting.
Shl,
/// Like `Shl`, but is UB if the RHS >= LHS::BITS
/// Like `Shl`, but is UB if the RHS >= LHS::BITS or RHS < 0
ShlUnchecked,
/// The `>>` operator (shift right)
///
/// The offset is truncated to the size of the first operand before shifting.
/// The offset is truncated to the size of the first operand and made unsigned before shifting.
///
/// This is an arithmetic shift if the LHS is signed
/// and a logical shift if the LHS is unsigned.
Shr,
/// Like `Shl`, but is UB if the RHS >= LHS::BITS
/// Like `Shl`, but is UB if the RHS >= LHS::BITS or RHS < 0
ShrUnchecked,
/// The `==` operator (equality)
Eq,

9
compiler/rustc_mir_build/src/build/expr/as_rvalue.rs
View File

@ -600,10 +600,12 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
BinOp::Shl | BinOp::Shr if self.check_overflow && ty.is_integral() => {
// 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.
// type and do that same comparison.
// A negative value will be *at least* 128 after the cast (that's i8::MIN),
// and 128 is an overflowing shift amount for all our currently existing types,
// so this cast can never make us miss an overflow.
let (lhs_size, _) = ty.int_size_and_signed(self.tcx);
assert!(lhs_size.bits() <= 128);
let rhs_ty = rhs.ty(&self.local_decls, self.tcx);
let (rhs_size, _) = rhs_ty.int_size_and_signed(self.tcx);
@ -625,7 +627,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
// 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,
unsigned_ty,

9
src/tools/miri/tests/fail/intrinsics/unchecked_shl2.rs Normal file
View File

@ -0,0 +1,9 @@
#![feature(core_intrinsics)]
use std::intrinsics;
fn main() {
unsafe {
let _n = intrinsics::unchecked_shl(1i8, -1);
//~^ ERROR: overflowing shift by -1 in `unchecked_shl`
}
}

15
src/tools/miri/tests/fail/intrinsics/unchecked_shl2.stderr Normal file
View File

@ -0,0 +1,15 @@
error: Undefined Behavior: overflowing shift by -1 in `unchecked_shl`
--> $DIR/unchecked_shl2.rs:LL:CC
|
LL | let _n = intrinsics::unchecked_shl(1i8, -1);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -1 in `unchecked_shl`
|
= help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
= help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
= note: BACKTRACE:
= note: inside `main` at $DIR/unchecked_shl2.rs:LL:CC
note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
error: aborting due to previous error

40
tests/ui/consts/const-int-unchecked.stderr
View File

@ -62,61 +62,61 @@ error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:41:33
|
LL | const SHL_I8_NEG: i8 = unsafe { intrinsics::unchecked_shl(5_i8, -1) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 255 in `unchecked_shl`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -1 in `unchecked_shl`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:43:35
|
LL | const SHL_I16_NEG: i16 = unsafe { intrinsics::unchecked_shl(5_16, -1) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 65535 in `unchecked_shl`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -1 in `unchecked_shl`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:45:35
|
LL | const SHL_I32_NEG: i32 = unsafe { intrinsics::unchecked_shl(5_i32, -1) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 4294967295 in `unchecked_shl`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -1 in `unchecked_shl`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:47:35
|
LL | const SHL_I64_NEG: i64 = unsafe { intrinsics::unchecked_shl(5_i64, -1) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 18446744073709551615 in `unchecked_shl`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -1 in `unchecked_shl`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:49:37
|
LL | const SHL_I128_NEG: i128 = unsafe { intrinsics::unchecked_shl(5_i128, -1) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 340282366920938463463374607431768211455 in `unchecked_shl`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -1 in `unchecked_shl`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:55:40
|
LL | const SHL_I8_NEG_RANDOM: i8 = unsafe { intrinsics::unchecked_shl(5_i8, -6) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 250 in `unchecked_shl`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -6 in `unchecked_shl`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:57:42
|
LL | const SHL_I16_NEG_RANDOM: i16 = unsafe { intrinsics::unchecked_shl(5_16, -13) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 65523 in `unchecked_shl`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -13 in `unchecked_shl`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:59:42
|
LL | const SHL_I32_NEG_RANDOM: i32 = unsafe { intrinsics::unchecked_shl(5_i32, -25) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 4294967271 in `unchecked_shl`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -25 in `unchecked_shl`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:61:42
|
LL | const SHL_I64_NEG_RANDOM: i64 = unsafe { intrinsics::unchecked_shl(5_i64, -30) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 18446744073709551586 in `unchecked_shl`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -30 in `unchecked_shl`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:63:44
|
LL | const SHL_I128_NEG_RANDOM: i128 = unsafe { intrinsics::unchecked_shl(5_i128, -93) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 340282366920938463463374607431768211363 in `unchecked_shl`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -93 in `unchecked_shl`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:70:29
@ -182,61 +182,61 @@ error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:96:33
|
LL | const SHR_I8_NEG: i8 = unsafe { intrinsics::unchecked_shr(5_i8, -1) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 255 in `unchecked_shr`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -1 in `unchecked_shr`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:98:35
|
LL | const SHR_I16_NEG: i16 = unsafe { intrinsics::unchecked_shr(5_16, -1) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 65535 in `unchecked_shr`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -1 in `unchecked_shr`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:100:35
|
LL | const SHR_I32_NEG: i32 = unsafe { intrinsics::unchecked_shr(5_i32, -1) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 4294967295 in `unchecked_shr`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -1 in `unchecked_shr`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:102:35
|
LL | const SHR_I64_NEG: i64 = unsafe { intrinsics::unchecked_shr(5_i64, -1) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 18446744073709551615 in `unchecked_shr`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -1 in `unchecked_shr`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:104:37
|
LL | const SHR_I128_NEG: i128 = unsafe { intrinsics::unchecked_shr(5_i128, -1) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 340282366920938463463374607431768211455 in `unchecked_shr`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -1 in `unchecked_shr`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:110:40
|
LL | const SHR_I8_NEG_RANDOM: i8 = unsafe { intrinsics::unchecked_shr(5_i8, -6) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 250 in `unchecked_shr`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -6 in `unchecked_shr`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:112:42
|
LL | const SHR_I16_NEG_RANDOM: i16 = unsafe { intrinsics::unchecked_shr(5_16, -13) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 65523 in `unchecked_shr`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -13 in `unchecked_shr`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:114:42
|
LL | const SHR_I32_NEG_RANDOM: i32 = unsafe { intrinsics::unchecked_shr(5_i32, -25) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 4294967271 in `unchecked_shr`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -25 in `unchecked_shr`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:116:42
|
LL | const SHR_I64_NEG_RANDOM: i64 = unsafe { intrinsics::unchecked_shr(5_i64, -30) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 18446744073709551586 in `unchecked_shr`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -30 in `unchecked_shr`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:118:44
|
LL | const SHR_I128_NEG_RANDOM: i128 = unsafe { intrinsics::unchecked_shr(5_i128, -93) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by 340282366920938463463374607431768211363 in `unchecked_shr`
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ overflowing shift by -93 in `unchecked_shr`
error[E0080]: evaluation of constant value failed
--> $DIR/const-int-unchecked.rs:123:25