T0b1/wasmtime
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

Remove MachInst::gen_constant (#5427)

Browse Source 
* aarch64: constant generation cleanup

Add support for MOVZ and MOVN generation via ISLE.
Handle f32const, f64const, and nop instructions via ISLE.
No longer call Inst::gen_constant from lower.rs.

* riscv64: constant generation cleanup

Handle f32const, f64const, and nop instructions via ISLE.

* s390x: constant generation cleanup

Fix rule priorities for "imm" term.
Only handle 32-bit stack offsets; no longer use load_constant64.

* x64: constant generation cleanup

No longer call Inst::gen_constant from lower.rs or abi.rs.

* Refactor LowerBackend::lower to return InstOutput

No longer write to the per-insn output registers; instead, return
an InstOutput vector of temp registers holding the outputs.

This will allow calling LowerBackend::lower multiple times for
the same instruction, e.g. to rematerialize constants.

When emitting the primary copy of the instruction during lowering,
writing to the per-insn registers is now done in lower_clif_block.

As a result, the ISLE lower_common routine is no longer needed.
In addition, the InsnOutput type and all code related to it
can be removed as well.

* Refactor IsleContext to hold a LowerBackend reference

Remove the "triple", "flags", and "isa_flags" fields that are
copied from LowerBackend to each IsleContext, and instead just
hold a reference to LowerBackend in IsleContext.

This will allow calling LowerBackend::lower from within callbacks
in src/machinst/isle.rs, e.g. to rematerialize constants.

To avoid having to pass LowerBackend references through multiple
functions, eliminate the lower_insn_to_regs subroutines in those
targets that still have them, and just inline into the main
lower routine.  This also eliminates lower_inst.rs on aarch64
and riscv64.

Replace all accesses to the removed IsleContext fields by going
through the LowerBackend reference.

* Remove MachInst::gen_constant

This addresses the problem described in issue
https://github.com/bytecodealliance/wasmtime/issues/4426
that targets currently have to duplicate code to emit
constants between the ISLE logic and the gen_constant
callback.

After the various cleanups in earlier patches in this series,
the only remaining user of get_constant is put_value_in_regs
in Lower.  This can now be removed, and instead constant
rematerialization can be performed in the put_in_regs ISLE
callback by simply directly calling LowerBackend::lower
on the instruction defining the constant (using a different
output register).

Since the check for egraph mode is now no longer performed in
put_value_in_regs, the Lower::flags member becomes obsolete.

Care needs to be taken that other calls directly to the
Lower::put_value_in_regs routine now handle the fact that
no more rematerialization is performed.  All such calls in
target code already historically handle constants themselves.
The remaining call site in the ISLE gen_call_common helper
can be redirected to the ISLE put_in_regs callback.

The existing target implementations of gen_constant are then
unused and can be removed.  (In some target there may still
be further opportunities to remove duplication between ISLE
and some local Rust code - this can be left to future patches.)
This commit is contained in:
Ulrich Weigand
2022-12-13 22:00:04 +01:00
committed by GitHub
parent 37ade17e2a
commit df923f18ca
45 changed files with 853 additions and 1481 deletions
Download Patch File Download Diff File Expand all files Collapse all files

78
cranelift/codegen/src/machinst/lower.rs
View File

@@ -14,9 +14,9 @@ use crate::ir::{
Type, Value, ValueDef, ValueLabelAssignments, ValueLabelStart,
};
use crate::machinst::{
non_writable_value_regs, writable_value_regs, BlockIndex, BlockLoweringOrder, Callee,
LoweredBlock, MachLabel, Reg, SigSet, VCode, VCodeBuilder, VCodeConstant, VCodeConstantData,
VCodeConstants, VCodeInst, ValueRegs, Writable,
writable_value_regs, BlockIndex, BlockLoweringOrder, Callee, LoweredBlock, MachLabel, Reg,
SigSet, VCode, VCodeBuilder, VCodeConstant, VCodeConstantData, VCodeConstants, VCodeInst,
ValueRegs, Writable,
};
use crate::{trace, CodegenResult};
use alloc::vec::Vec;
@@ -26,6 +26,9 @@ use std::fmt::Debug;
use super::{VCodeBuildDirection, VRegAllocator};
/// A vector of ValueRegs, used to represent the outputs of an instruction.
pub type InstOutput = SmallVec<[ValueRegs<Reg>; 2]>;
/// An "instruction color" partitions CLIF instructions by side-effecting ops.
/// All instructions with the same "color" are guaranteed not to be separated by
/// any side-effecting op (for this purpose, loads are also considered
@@ -121,7 +124,7 @@ pub trait LowerBackend {
/// edge (block-param actuals) into registers, because the actual branch
/// generation (`lower_branch_group()`) happens *after* any possible merged
/// out-edge.
fn lower(&self, ctx: &mut Lower<Self::MInst>, inst: Inst) -> CodegenResult<()>;
fn lower(&self, ctx: &mut Lower<Self::MInst>, inst: Inst) -> CodegenResult<InstOutput>;
/// Lower a block-terminating group of branches (which together can be seen
/// as one N-way branch), given a vcode MachLabel for each target.
@@ -146,9 +149,6 @@ pub struct Lower<'func, I: VCodeInst> {
/// The function to lower.
f: &'func Function,
/// Machine-independent flags.
flags: crate::settings::Flags,
/// The set of allocatable registers.
allocatable: PRegSet,
@@ -324,7 +324,6 @@ impl<'func, I: VCodeInst> Lower<'func, I> {
/// Prepare a new lowering context for the given IR function.
pub fn new(
f: &'func Function,
flags: crate::settings::Flags,
machine_env: &MachineEnv,
abi: Callee<I::ABIMachineSpec>,
emit_info: I::Info,
@@ -415,7 +414,6 @@ impl<'func, I: VCodeInst> Lower<'func, I> {
Ok(Lower {
f,
flags,
allocatable: PRegSet::from(machine_env),
vcode,
vregs,
@@ -742,7 +740,27 @@ impl<'func, I: VCodeInst> Lower<'func, I> {
// or any of its outputs its used.
if has_side_effect || value_needed {
trace!("lowering: inst {}: {:?}", inst, self.f.dfg[inst]);
backend.lower(self, inst)?;
let temp_regs = backend.lower(self, inst)?;
// The ISLE generated code emits its own registers to define the
// instruction's lowered values in. However, other instructions
// that use this SSA value will be lowered assuming that the value
// is generated into a pre-assigned, different, register.
//
// To connect the two, we set up "aliases" in the VCodeBuilder
// that apply when it is building the Operand table for the
// regalloc to use. These aliases effectively rewrite any use of
// the pre-assigned register to the register that was returned by
// the ISLE lowering logic.
debug_assert_eq!(temp_regs.len(), self.num_outputs(inst));
for i in 0..self.num_outputs(inst) {
let regs = temp_regs[i];
let dsts = self.value_regs[self.f.dfg.inst_results(inst)[i]];
debug_assert_eq!(regs.len(), dsts.len());
for (dst, temp) in dsts.regs().iter().zip(regs.regs().iter()) {
self.set_vreg_alias(*dst, *temp);
}
}
}
let loc = self.srcloc(inst);
@@ -1249,33 +1267,6 @@ impl<'func, I: VCodeInst> Lower<'func, I> {
assert!(!self.inst_sunk.contains(&inst));
}
// If the value is a constant, then (re)materialize it at each
// use. This lowers register pressure. (Only do this if we are
// not using egraph-based compilation; the egraph framework
// more efficiently rematerializes constants where needed.)
if !self.flags.use_egraphs() {
if let Some(c) = self
.f
.dfg
.value_def(val)
.inst()
.and_then(|inst| self.get_constant(inst))
{
let ty = self.f.dfg.value_type(val);
let regs = self.alloc_tmp(ty);
trace!(" -> regs {:?}", regs);
assert!(regs.is_valid());
let insts = I::gen_constant(regs, c.into(), ty, |ty| {
self.alloc_tmp(ty).only_reg().unwrap()
});
for inst in insts {
self.emit(inst);
}
return non_writable_value_regs(regs);
}
}
let regs = self.value_regs[val];
trace!(" -> regs {:?}", regs);
assert!(regs.is_valid());
@@ -1284,19 +1275,6 @@ impl<'func, I: VCodeInst> Lower<'func, I> {
regs
}
/// Get the `idx`th output register(s) of the given IR instruction.
///
/// When `backend.lower_inst_to_regs(ctx, inst)` is called, it is expected
/// that the backend will write results to these output register(s). This
/// register will always be "fresh"; it is guaranteed not to overlap with
/// any of the inputs, and can be freely used as a scratch register within
/// the lowered instruction sequence, as long as its final value is the
/// result of the computation.
pub fn get_output(&self, ir_inst: Inst, idx: usize) -> ValueRegs<Writable<Reg>> {
let val = self.f.dfg.inst_results(ir_inst)[idx];
writable_value_regs(self.value_regs[val])
}
}
/// Codegen primitives: allocate temps, emit instructions, set result registers,