Cranelift: add a config option for alias analysis and redundant-load …

…elimination. (#4349)

This allows for experiments as in here [1] and also generally gives an
option to anyone who is concerned that the extra optimization may be
counterproductive or take too much time. The optimization remains
enabled by default.

[1]
#4163 (comment)

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

@@ -31,6 +31,16 @@ pub(crate) fn define() -> SettingGroup {
         vec!["none", "speed", "speed_and_size"],
     );
 
+    settings.add_bool(
+        "enable_alias_analysis",
+        "Do redundant-load optimizations with alias analysis.",
+        r#"
+            This enables the use of a simple alias analysis to optimize away redundant loads.
+            Only effective when `opt_level` is `speed` or `speed_and_size`.
+        "#,
+        true,
+    );
+
     settings.add_bool(
         "enable_verifier",
         "Run the Cranelift IR verifier at strategic times during compilation.",

cranelift/codegen/src/context.rs

@@ -163,7 +163,7 @@ impl Context {
 
         self.remove_constant_phis(isa)?;
 
-        if opt_level != OptLevel::None {
+        if opt_level != OptLevel::None && isa.flags().enable_alias_analysis() {
             self.replace_redundant_loads()?;
             self.simple_gvn(isa)?;
         }

cranelift/codegen/src/settings.rs

@@ -516,6 +516,7 @@ libcall_call_conv = "isa_default"
 baldrdash_prologue_words = 0
 probestack_size_log2 = 12
 regalloc_checker = false
+enable_alias_analysis = true
 enable_verifier = true
 is_pic = false
 use_colocated_libcalls = false

crates/wasmtime/src/engine.rs

@@ -384,6 +384,7 @@ impl Engine {
             | "machine_code_cfg_info"
             | "tls_model" // wasmtime doesn't use tls right now
             | "opt_level" // opt level doesn't change semantics
+            | "enable_alias_analysis" // alias analysis-based opts don't change semantics
             | "probestack_func_adjusts_sp" // probestack above asserted disabled
             | "probestack_size_log2" // probestack above asserted disabled
             | "regalloc" // shouldn't change semantics