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stage2: Teach Liveness that safety checks do not modify memory

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This change adds to Liveness a simple pattern match for the
try-like `.condbr` blocks emitted by Sema's safety checks. This
allows us to determine that these do not modify memory, which
permits us to elide additional loads in the backend.

As @Vexu points out in the main issue, this is probably not a
complete solution on its own. We'll still want a way to reliably
narrow the load/copy when performing several consecutive accesses,
such as `foo.arr[x][y].z`

Resolves https://github.com/ziglang/zig/issues/12215
This commit is contained in:
Cody Tapscott 2022-10-05 05:34:42 -07:00 committed by Andrew Kelley
parent d0eef26687
commit 25d3713b07
1 changed files with 35 additions and 0 deletions
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35
src/Liveness.zig
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@ -501,6 +501,41 @@ pub fn categorizeOperand(
return .complex;
},
.block => {
const extra = air.extraData(Air.Block, air_datas[inst].ty_pl.payload);
const body = air.extra[extra.end..][0..extra.data.body_len];
if (body.len == 1 and air_tags[body[0]] == .cond_br) {
// Peephole optimization for "panic-like" conditionals, which have
// one empty branch and another which calls a `noreturn` function.
// This allows us to infer that safety checks do not modify memory,
// as far as control flow successors are concerned.
const inst_data = air_datas[body[0]].pl_op;
const cond_extra = air.extraData(Air.CondBr, inst_data.payload);
if (inst_data.operand == operand_ref and operandDies(l, body[0], 0))
return .tomb;
if (cond_extra.data.then_body_len != 1 or cond_extra.data.else_body_len != 1)
return .complex;
var operand_live: bool = true;
for (air.extra[cond_extra.end..][0..2]) |cond_inst| {
if (l.categorizeOperand(air, cond_inst, operand) == .tomb)
operand_live = false;
switch (air_tags[cond_inst]) {
.br => { // Breaks immediately back to block
const br = air_datas[cond_inst].br;
if (br.block_inst != inst)
return .complex;
},
.call => {}, // Calls a noreturn function
else => return .complex,
}
}
return if (operand_live) .none else .tomb;
}
return .complex;
},
.@"try" => {