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Rollup merge of #117832 - RalfJung:interpret-shift, r=cjgillot

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

While we're at it, also update comments in codegen and MIR building related to shifts, and fix the overflow error printed by Miri on negative shift amounts.
This commit is contained in:
Michael Goulet 2023-11-19 19:14:33 -08:00 • committed by GitHub
commit 94d9b7e708
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7 changed files with 86 additions and 58 deletions
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9
compiler/rustc_mir_build/src/build/expr/as_rvalue.rs
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@ -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,