Enable egraph-based optimization by default. (#5587)

This PR follows up on #5382 and #5391, which rebuilt the egraph-based optimization framework to be more performant, by enabling it by default.

Based on performance results in #5382 (my measurements on SpiderMonkey and bjorn3's independent confirmation with cg_clif), it seems that this is reasonable to enable. Now that we have been fuzzing compiler configurations with egraph opts (#5388) for 6 weeks, having fixed a few fuzzbugs that came up (#5409, #5420, #5438) and subsequently received no further reports from OSS-Fuzz, I believe it is stable enough to rely on.

This PR enables `use_egraphs`, and also normalizes its meaning: previously it forced optimization (it basically meant "turn on the egraph optimization machinery"), now it runs egraph opts if the opt level indicates (it means "use egraphs to optimize if we are going to optimize"). The conditionals in the top-level pass driver are a little subtle, but will get simpler once we can remove the non-egraph path (which we plan to do eventually!).

Fixes #5181.

cranelift/codegen/meta/src/shared/settings.rs

@@ -58,10 +58,12 @@ pub(crate) fn define() -> SettingGroup {
         "Enable egraph-based optimization.",
         r#"
             This enables an optimization phase that converts CLIF to an egraph (equivalence graph)
-            representation, performs various rewrites, and then converts it back. This can result in
-            better optimization, but is currently considered experimental.
+            representation, performs various rewrites, and then converts it back. This should result in
+            better optimization, but the traditional optimization pass structure is also still
+            available by setting this to `false`. The `false` setting will eventually be
+            deprecated and removed.
         "#,
-        false,
+        true,
     );
 
     settings.add_bool(

cranelift/codegen/src/context.rs

@@ -185,18 +185,20 @@ impl Context {
         self.compute_domtree();
         self.eliminate_unreachable_code(isa)?;
 
-        if isa.flags().use_egraphs() || opt_level != OptLevel::None {
+        if opt_level != OptLevel::None {
             self.dce(isa)?;
         }
 
         self.remove_constant_phis(isa)?;
 
-        if isa.flags().use_egraphs() {
-            self.egraph_pass()?;
-        } else if opt_level != OptLevel::None && isa.flags().enable_alias_analysis() {
-            for _ in 0..2 {
-                self.replace_redundant_loads()?;
-                self.simple_gvn(isa)?;
+        if opt_level != OptLevel::None {
+            if isa.flags().use_egraphs() {
+                self.egraph_pass()?;
+            } else if isa.flags().enable_alias_analysis() {
+                for _ in 0..2 {
+                    self.replace_redundant_loads()?;
+                    self.simple_gvn(isa)?;
+                }
             }
         }
 

cranelift/codegen/src/machinst/isle.rs

@@ -16,7 +16,7 @@ pub use crate::machinst::{
     ABIArg, ABIArgSlot, InputSourceInst, Lower, LowerBackend, RealReg, Reg, RelocDistance, Sig,
     VCodeInst, Writable,
 };
-pub use crate::settings::TlsModel;
+pub use crate::settings::{OptLevel, TlsModel};
 
 pub type Unit = ();
 pub type ValueSlice = (ValueList, usize);
@@ -131,7 +131,9 @@ macro_rules! isle_lower_prelude_methods {
             // 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.backend.flags().use_egraphs() {
+            if !(self.backend.flags().use_egraphs()
+                && self.backend.flags().opt_level() != OptLevel::None)
+            {
                 let inputs = self.lower_ctx.get_value_as_source_or_const(val);
                 if inputs.constant.is_some() {
                     let insn = match inputs.inst {

cranelift/codegen/src/settings.rs

@@ -528,7 +528,7 @@ probestack_strategy = "outline"
 regalloc_checker = false
 regalloc_verbose_logs = false
 enable_alias_analysis = true
-use_egraphs = false
+use_egraphs = true
 enable_verifier = true
 is_pic = false
 use_colocated_libcalls = false

cranelift/filetests/filetests/egraph/algebraic.clif

@@ -1,5 +1,5 @@
 test optimize
-set opt_level=none
+set opt_level=speed
 set use_egraphs=true
 target x86_64
 

cranelift/filetests/filetests/egraph/alias_analysis.clif

@@ -1,5 +1,5 @@
 test optimize
-set opt_level=none
+set opt_level=speed
 set use_egraphs=true
 target x86_64
 

cranelift/filetests/filetests/egraph/basic-gvn.clif

@@ -1,5 +1,5 @@
 test optimize
-set opt_level=none
+set opt_level=speed
 set use_egraphs=true
 target x86_64
 

cranelift/filetests/filetests/egraph/cprop.clif

@@ -1,5 +1,5 @@
 test optimize
-set opt_level=none
+set opt_level=speed
 set use_egraphs=true
 target x86_64
 

cranelift/filetests/filetests/egraph/i128-opts.clif

@@ -1,5 +1,5 @@
 test optimize
-set opt_level=speed_and_size
+set opt_level=speed
 set use_egraphs=true
 target x86_64
 

cranelift/filetests/filetests/egraph/isplit.clif

@@ -1,6 +1,6 @@
 test interpret
 test run
-set opt_level=speed_and_size
+set opt_level=speed
 set use_egraphs=true
 set enable_llvm_abi_extensions=true
 target x86_64

cranelift/filetests/filetests/egraph/issue-5405.clif

@@ -1,6 +1,6 @@
 test interpret
 test run
-set opt_level=speed_and_size
+set opt_level=speed
 set use_egraphs=true
 target aarch64
 

cranelift/filetests/filetests/egraph/issue-5437.clif

@@ -1,4 +1,5 @@
 test compile
+set opt_level=speed
 set use_egraphs=true
 target x86_64
 target aarch64

cranelift/filetests/filetests/egraph/licm.clif

@@ -1,5 +1,5 @@
 test optimize
-set opt_level=none
+set opt_level=speed
 set use_egraphs=true
 target x86_64
 

cranelift/filetests/filetests/egraph/misc.clif

@@ -1,5 +1,5 @@
 test optimize
-set opt_level=none
+set opt_level=speed
 set use_egraphs=true
 target x86_64
 

cranelift/filetests/filetests/egraph/multivalue.clif

@@ -1,4 +1,5 @@
 test compile precise-output
+set opt_level=speed
 set use_egraphs=true
 target x86_64
 

cranelift/filetests/filetests/egraph/not_a_load.clif

@@ -1,4 +1,5 @@
 test compile precise-output
+set opt_level=speed
 set use_egraphs=true
 target x86_64
 

cranelift/filetests/filetests/egraph/remat.clif

@@ -1,5 +1,5 @@
 test optimize
-set opt_level=none
+set opt_level=speed
 set use_egraphs=true
 target x86_64
 

cranelift/filetests/filetests/wasm/duplicate-loads-dynamic-memory.wat

@@ -4,7 +4,8 @@
 ;;!
 ;;! settings = [
 ;;!   "enable_heap_access_spectre_mitigation=true",
-;;!   "opt_level=speed_and_size"
+;;!   "opt_level=speed_and_size",
+;;!   "use_egraphs=false"
 ;;! ]
 ;;!
 ;;! [globals.vmctx]
@@ -112,4 +113,4 @@
 ;;
 ;;                                 block1:
 ;; @006e                               return v14, v14
-;; }
+;; }

cranelift/filetests/filetests/wasm/duplicate-loads-static-memory.wat

@@ -4,7 +4,8 @@
 ;;!
 ;;! settings = [
 ;;!   "enable_heap_access_spectre_mitigation=true",
-;;!   "opt_level=speed_and_size"
+;;!   "opt_level=speed_and_size",
+;;!   "use_egraphs=false"
 ;;! ]
 ;;!
 ;;! [globals.vmctx]

crates/wasmtime/src/config.rs

@@ -832,14 +832,13 @@ impl Config {
     /// Configures the Cranelift code generator to use its
     /// "egraph"-based mid-end optimizer.
     ///
-    /// This optimizer is intended to replace the compiler's more
-    /// traditional pipeline of optimization passes with a unified
-    /// code-rewriting system. It is not yet on by default, but it is
-    /// intended to become the default in a future version. It may
-    /// result in faster code, at the cost of slightly more
-    /// compilation-time work.
-    ///
-    /// The default value for this is `false`.
+    /// This optimizer has replaced the compiler's more traditional
+    /// pipeline of optimization passes with a unified code-rewriting
+    /// system. It is on by default, but the traditional optimization
+    /// pass structure is still available for now (it is deprecrated and
+    /// will be removed in a future version).
+    ///
+    /// The default value for this is `true`.
     #[cfg(compiler)]
     #[cfg_attr(nightlydoc, doc(cfg(feature = "cranelift")))] // see build.rs
     pub fn cranelift_use_egraphs(&mut self, enable: bool) -> &mut Self {