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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_codegen_ssa/src/base.rs
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@ -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