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Make omission of impossible if-else branches work with constants

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Until now, we only optimized away impossible branches when there is a
literal true/false in the code. But since the LLVM IR builder already does
constant folding for us, we can trivially expand that to work with
constants as well.

Refs #7834
This commit is contained in:
Björn Steinbrink 2013-07-25 10:53:27 +02:00
parent 75a08622e8
commit e0685e22a3
1 changed files with 27 additions and 33 deletions
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60
src/librustc/middle/trans/controlflow.rs
View file

@ -65,44 +65,38 @@ pub fn trans_if(bcx: @mut Block,
let _icx = push_ctxt("trans_if");
match cond.node {
// `if true` and `if false` can be trans'd more efficiently,
// by dropping branches that are known to be impossible.
ast::expr_lit(@ref l) => match l.node {
ast::lit_bool(true) => {
// if true { .. } [else { .. }]
let then_bcx_in = scope_block(bcx, thn.info(), "if_true_then");
let then_bcx_out = trans_block(then_bcx_in, thn, dest);
let then_bcx_out = trans_block_cleanups(then_bcx_out,
block_cleanups(then_bcx_in));
Br(bcx, then_bcx_in.llbb);
return then_bcx_out;
}
ast::lit_bool(false) => {
match els {
// if false { .. } else { .. }
Some(elexpr) => {
let (else_bcx_in, else_bcx_out) =
trans_if_else(bcx, elexpr, dest, "if_false_else");
Br(bcx, else_bcx_in.llbb);
return else_bcx_out;
}
// if false { .. }
None => return bcx,
}
}
_ => {}
},
_ => {}
}
let Result {bcx, val: cond_val} =
expr::trans_to_datum(bcx, cond).to_result();
let then_bcx_in = scope_block(bcx, thn.info(), "then");
let cond_val = bool_to_i1(bcx, cond_val);
// Drop branches that are known to be impossible
if is_const(cond_val) && !is_undef(cond_val) {
if const_to_uint(cond_val) == 1 {
// if true { .. } [else { .. }]
let then_bcx_in = scope_block(bcx, thn.info(), "if_true_then");
let then_bcx_out = trans_block(then_bcx_in, thn, dest);
let then_bcx_out = trans_block_cleanups(then_bcx_out,
block_cleanups(then_bcx_in));
Br(bcx, then_bcx_in.llbb);
return then_bcx_out;
} else {
match els {
// if false { .. } else { .. }
Some(elexpr) => {
let (else_bcx_in, else_bcx_out) =
trans_if_else(bcx, elexpr, dest, "if_false_else");
Br(bcx, else_bcx_in.llbb);
return else_bcx_out;
}
// if false { .. }
None => return bcx,
}
}
}
let then_bcx_in = scope_block(bcx, thn.info(), "then");
let then_bcx_out = trans_block(then_bcx_in, thn, dest);
let then_bcx_out = trans_block_cleanups(then_bcx_out,
block_cleanups(then_bcx_in));