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remove assignments to ZST places

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This commit is contained in:
Erik Desjardins 2021-03-15 21:57:52 -04:00
parent affb081fa0
commit 899836306f
1 changed files with 26 additions and 1 deletions
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27
compiler/rustc_mir/src/transform/instcombine.rs
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@ -12,10 +12,12 @@
impl<'tcx> MirPass<'tcx> for InstCombine {
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
let param_env = tcx.param_env(body.source.def_id());
let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
let ctx = InstCombineContext { tcx, local_decls };
let ctx = InstCombineContext { tcx, local_decls, param_env };
for block in basic_blocks.iter_mut() {
for statement in block.statements.iter_mut() {
ctx.combine_zst(&statement.source_info, &mut statement.kind);
match statement.kind {
StatementKind::Assign(box (_place, ref mut rvalue)) => {
ctx.combine_bool_cmp(&statement.source_info, rvalue);
@ -32,6 +34,7 @@ fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
struct InstCombineContext<'tcx, 'a> {
tcx: TyCtxt<'tcx>,
local_decls: &'a LocalDecls<'tcx>,
param_env: ty::ParamEnv<'tcx>,
}
impl<'tcx, 'a> InstCombineContext<'tcx, 'a> {
@ -41,6 +44,28 @@ fn should_combine(&self, source_info: &SourceInfo, rvalue: &Rvalue<'tcx>) -> boo
})
}
/// Remove assignments to inhabited ZST places.
fn combine_zst(&self, source_info: &SourceInfo, kind: &mut StatementKind<'tcx>) {
match kind {
StatementKind::Assign(box (place, _)) => {
let place_ty = place.ty(self.local_decls, self.tcx).ty;
if let Ok(layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
if layout.is_zst() && !layout.abi.is_uninhabited() {
if self.tcx.consider_optimizing(|| {
format!(
"InstCombine ZST - Place: {:?} SourceInfo: {:?}",
place, source_info
)
}) {
*kind = StatementKind::Nop;
}
}
}
}
_ => {}
}
}
/// Transform boolean comparisons into logical operations.
fn combine_bool_cmp(&self, source_info: &SourceInfo, rvalue: &mut Rvalue<'tcx>) {
match rvalue {