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Improve code emitted for inserting padding before unsized field.

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Hat-tip to eddyb for the appropriate bit-trickery here.
This commit is contained in:
Felix S. Klock II 2015-07-29 22:18:39 +02:00
parent 26f4ebe7a0
commit 21be09448b
1 changed files with 5 additions and 13 deletions

18
src/librustc_trans/trans/glue.rs

@ -475,21 +475,13 @@ pub fn size_and_align_of_dst<'blk, 'tcx>(bcx: Block<'blk, 'tcx>, t: Ty<'tcx>, in
//
// `size + ((size & (align-1)) ? align : 0)`
//
// Currently I am emulating the above via:
// emulated via the semi-standard fast bit trick:
//
// `size + ((size & (align-1)) * align-(size & (align-1)))`
//
// because I am not sure which is cheaper between a branch
// or a multiply.
// `(size + (align-1)) & !align`
let mask = Sub(bcx, align, C_uint(bcx.ccx(), 1_u64), dbloc);
let lowbits = And(bcx, size, mask, DebugLoc::None);
let nonzero = ICmp(bcx, llvm::IntNE, lowbits, C_uint(bcx.ccx(), 0_u64), dbloc);
let add_size = Mul(bcx,
ZExt(bcx, nonzero, Type::i64(bcx.ccx())),
Sub(bcx, align, lowbits, dbloc),
dbloc);
let size = Add(bcx, size, add_size, dbloc);
let addend = Sub(bcx, align, C_uint(bcx.ccx(), 1_u64), dbloc);
let size = And(
bcx, Add(bcx, size, addend, dbloc), Neg(bcx, align, dbloc), dbloc);
(size, align)
}