Revert "Disable recompilation of existing Scripts from Isolate compilation cache"

This reverts commit a7607221.

Reason for revert: fails on GC Stress bot:
https://ci.chromium.org/ui/p/v8/builders/ci/V8%20Linux%20-%20gc%20stress/38512/overview

Original change's description:
> Disable recompilation of existing Scripts from Isolate compilation cache
>
> My previous change https://crrev.com/c/3597106 led to some performance
> regressions in time spent on parsing and compilation. This change
> disables the ability to recompile an existing uncompiled Script, as an
> attempt to both fix the regressions and isolate which part of the
> previous change was the cause of those problems.
>
> Bug: v8:12808, chromium:1325566, chromium:1325567, chromium:1325601
> Change-Id: Ifa086bf27070da8f4b3c0e4415af5ca7b6706b0a
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3652252
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
> Cr-Commit-Position: refs/heads/main@{#80616}

Bug: v8:12808, chromium:1325566, chromium:1325567, chromium:1325601
Change-Id: I0f56163856d04ff49da96b0fb344fb59b5501a40
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3653273
Auto-Submit: Adam Klein <adamk@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#80617}

src/codegen/compiler.cc

@@ -3043,9 +3043,7 @@ MaybeHandle<SharedFunctionInfo> GetSharedFunctionInfoForScriptImpl(
     // First check per-isolate compilation cache.
     CompilationCacheScript::LookupResult lookup_result =
         compilation_cache->LookupScript(source, script_details, language_mode);
-    if (FLAG_isolate_script_cache_recompilation) {
-      maybe_script = lookup_result.script();
-    }
+    maybe_script = lookup_result.script();
     maybe_result = lookup_result.toplevel_sfi();
     is_compiled_scope = lookup_result.is_compiled_scope();
     if (!maybe_result.is_null()) {

src/flags/flag-definitions.h

@@ -959,9 +959,6 @@ DEFINE_BOOL(turbo_collect_feedback_in_generic_lowering, true,
             "enable experimental feedback collection in generic lowering.")
 DEFINE_BOOL(isolate_script_cache_ageing, true,
             "enable ageing of the isolate script cache.")
-DEFINE_BOOL(isolate_script_cache_recompilation, false,
-            "enable recompiling an existing Script from the Isolate cache when "
-            "there is not a compiled top-level SharedFunctionInfo")
 
 DEFINE_FLOAT(script_delay, 0, "busy wait [ms] on every Script::Run")
 DEFINE_FLOAT(script_delay_once, 0, "busy wait [ms] on the first Script::Run")

test/cctest/heap/test-heap.cc

@@ -1578,12 +1578,6 @@ void CompilationCacheRegeneration(bool retain_root_sfi, bool flush_root_sfi,
     return;
   }
 
-  // If the compiler is configured to not recompile a flushed root SFI, then
-  // this test is invalid.
-  if (flush_root_sfi && !FLAG_isolate_script_cache_recompilation) {
-    return;
-  }
-
   // TODO(v8:12808): Remove this check once background compilation is capable of
   // reusing an existing Script.
   if (flush_root_sfi && FLAG_stress_background_compile) {

test/cctest/test-api.cc

@@ -23951,12 +23951,6 @@ TEST(StreamingWithIsolateScriptCacheClearingRootSFI) {
     return;
   }
 
-  // If the compiler is configured to not recompile a flushed root SFI, then
-  // this test is invalid.
-  if (!v8::internal::FLAG_isolate_script_cache_recompilation) {
-    return;
-  }
-
   StreamingWithIsolateScriptCache(true);
 }