[Custom Descriptors] Use exact imports in wasm-split (#8043)

When custom descriptors are enabled, import primary functions into
secondary modules using exact imports. This allows references to those
functions in the secondary module to remain exact, just like they are
before splitting. Remove the exact casts we previously inserted to fix
this problem.

Do not use exact imports when custom descriptors are not enabled. In
principle this could cause validation errors because we allow e.g. exact
locals even when custom descriptors are not enabled. This could be
worked around in the future either by running a pass to remove exactness
before splitting or by always using and allowing exact imports, but then
emitting them as inexact imports when custom descriptors are disabled.

Author: tlively

src/ir/module-splitting.cpp

@@ -67,7 +67,13 @@
 //   2. It assumes that either all table segment offsets are constants or there
 //      is exactly one segment that may have a non-constant offset. It also
 //      assumes that all segments are active segments.
-
+//
+//   3. It assumes that if exact function references are required for validity
+//      (because they are stored in a local with an exact function type, for
+//      example), then custom descriptors are allowed so primary functions can
+//      be imported exactly. This could be worked around by removing exactness
+//      from the IR before splitting.
+//
 #include "ir/module-splitting.h"
 #include "asmjs/shared-constants.h"
 #include "ir/export-utils.h"
@@ -341,7 +347,6 @@ struct ModuleSplitter {
   void setupTablePatching();
   void shareImportableItems();
   void removeUnusedSecondaryElements();
-  void updateIR();
 
   ModuleSplitter(Module& primary, const Config& config)
     : config(config), primary(primary), tableManager(primary),
@@ -358,7 +363,6 @@ struct ModuleSplitter {
     setupTablePatching();
     shareImportableItems();
     removeUnusedSecondaryElements();
-    updateIR();
   }
 };
 
@@ -520,7 +524,7 @@ void ModuleSplitter::exportImportFunction(Name funcName,
       func->hasExplicitName = primaryFunc->hasExplicitName;
       func->module = config.importNamespace;
       func->base = exportName;
-      func->type = func->type.with(Inexact);
+      func->type = func->type.withInexactIfNoCustomDescs(secondary->features);
       secondary->addFunction(std::move(func));
     }
   }
@@ -981,31 +985,6 @@ void ModuleSplitter::removeUnusedSecondaryElements() {
   }
 }
 
-void ModuleSplitter::updateIR() {
-  // Imported functions may need type updates.
-  struct Fixer : public PostWalker<Fixer> {
-    void visitRefFunc(RefFunc* curr) {
-      auto& wasm = *getModule();
-      auto* func = wasm.getFunction(curr->func);
-      if (func->type != curr->type) {
-        // This became an import, and lost exactness.
-        assert(!func->type.isExact());
-        assert(curr->type.isExact());
-        if (wasm.features.hasCustomDescriptors()) {
-          // Add a cast, as the parent may depend on the exactness to validate.
-          // TODO: The cast may be needed even without CD enabled
-          replaceCurrent(Builder(wasm).makeRefCast(curr, curr->type));
-        }
-        curr->type = curr->type.with(Inexact);
-      }
-    }
-  } fixer;
-  fixer.walkModule(&primary);
-  for (auto& secondaryPtr : secondaries) {
-    fixer.walkModule(secondaryPtr.get());
-  }
-}
-
 } // anonymous namespace
 
 Results splitFunctions(Module& primary, const Config& config) {

src/ir/module-splitting.h

@@ -26,14 +26,18 @@
 // functions. The secondary module imports all of its dependencies from the
 // primary module.
 //
-// This code currently makes a couple assumptions about the modules that will be
+// This code currently makes a few assumptions about the modules that will be
 // split and will fail assertions if those assumptions are not true.
 //
 //   1) It assumes that mutable-globals are allowed.
 //
 //   2) It assumes that either all segment offsets are constants or there is
 //      exactly one segment that may have a non-constant offset.
 //
+//   3) It assumes that exact function references are only required for validity
+//      if custom descriptors (and its exact function import feature) are
+//      allowed.
+//
 // These requirements will be relaxed as necessary in the future, but for now
 // this code should be considered experimental and used with care.
 

test/example/module-splitting.txt

@@ -745,7 +745,7 @@ Secondary:
 (module
  (type $0 (func))
  (import "primary" "%table" (table $0 1 funcref))
- (import "primary" "%foo_1" (func $foo (type $0)))
+ (import "primary" "%foo_1" (func $foo (exact (type $0))))
  (elem $0 (i32.const 0) $bar)
  (func $bar (type $0)
   (call $foo)
@@ -1055,7 +1055,7 @@ Secondary:
 (module
  (type $0 (func))
  (import "primary" "%table_2" (table $0 1 funcref))
- (import "primary" "%foo" (func $foo (type $0)))
+ (import "primary" "%foo" (func $foo (exact (type $0))))
  (elem $0 (i32.const 0) $bar)
  (func $bar (type $0)
   (call $foo)
@@ -1102,7 +1102,7 @@ Secondary:
 (module
  (type $0 (func (param i32) (result i32)))
  (import "primary" "%table_2" (table $0 1 funcref))
- (import "primary" "%foo" (func $foo (type $0) (param i32) (result i32)))
+ (import "primary" "%foo" (func $foo (exact (type $0) (param i32) (result i32))))
  (elem $0 (i32.const 0) $bar)
  (func $bar (type $0) (param $0 i32) (result i32)
   (call $foo

test/lit/wasm-split/exact.wast

@@ -73,7 +73,7 @@
 
  ;; SECONDARY:      (import "primary" "table" (table $timport$0 1 funcref))
 
- ;; SECONDARY:      (import "primary" "foo" (func $foo (type $0)))
+ ;; SECONDARY:      (import "primary" "foo" (func $foo (exact (type $0))))
 
  ;; SECONDARY:      (elem $0 (i32.const 0) $bar)
 
@@ -82,9 +82,7 @@
  ;; SECONDARY:      (func $bar (type $0)
  ;; SECONDARY-NEXT:  (local $exact (ref (exact $0)))
  ;; SECONDARY-NEXT:  (local.set $exact
- ;; SECONDARY-NEXT:   (ref.cast (ref (exact $0))
- ;; SECONDARY-NEXT:    (ref.func $foo)
- ;; SECONDARY-NEXT:   )
+ ;; SECONDARY-NEXT:   (ref.func $foo)
  ;; SECONDARY-NEXT:  )
  ;; SECONDARY-NEXT:  (local.set $exact
  ;; SECONDARY-NEXT:   (ref.func $bar)

test/lit/wasm-split/multi-split-escape-names.wast

@@ -24,9 +24,9 @@
 
  ;; MOD1:      (import "primary" "table" (table $timport$0 3 funcref))
 
- ;; MOD1:      (import "primary" "trampoline_std::operator<<\\28std::__2::basic_ostream<char\\2c\\20std::__2::char_traits<char>>&\\2c\\20wasm::Module&\\29" (func $trampoline_std::operator<<\28std::__2::basic_ostream<char\2c\20std::__2::char_traits<char>>&\2c\20wasm::Module&\29 (result f32)))
+ ;; MOD1:      (import "primary" "trampoline_std::operator<<\\28std::__2::basic_ostream<char\\2c\\20std::__2::char_traits<char>>&\\2c\\20wasm::Module&\\29" (func $trampoline_std::operator<<\28std::__2::basic_ostream<char\2c\20std::__2::char_traits<char>>&\2c\20wasm::Module&\29 (exact (result f32))))
 
- ;; MOD1:      (import "primary" "trampoline_wasm::Literal::Literal\\28std::__2::array<wasm::Literal\\2c\\204ul>\\20const&\\29" (func $trampoline_wasm::Literal::Literal\28std::__2::array<wasm::Literal\2c\204ul>\20const&\29 (result i64)))
+ ;; MOD1:      (import "primary" "trampoline_wasm::Literal::Literal\\28std::__2::array<wasm::Literal\\2c\\204ul>\\20const&\\29" (func $trampoline_wasm::Literal::Literal\28std::__2::array<wasm::Literal\2c\204ul>\20const&\29 (exact (result i64))))
 
  ;; MOD1:      (elem $0 (i32.const 2) $wasm::Type::getFeatures\28\29\20const)
 
@@ -38,16 +38,12 @@
  ;; MOD1-NEXT:  )
  ;; MOD1-NEXT:  (drop
  ;; MOD1-NEXT:   (call_ref $1
- ;; MOD1-NEXT:    (ref.cast (ref (exact $1))
- ;; MOD1-NEXT:     (ref.func $trampoline_wasm::Literal::Literal\28std::__2::array<wasm::Literal\2c\204ul>\20const&\29)
- ;; MOD1-NEXT:    )
+ ;; MOD1-NEXT:    (ref.func $trampoline_wasm::Literal::Literal\28std::__2::array<wasm::Literal\2c\204ul>\20const&\29)
  ;; MOD1-NEXT:   )
  ;; MOD1-NEXT:  )
  ;; MOD1-NEXT:  (drop
  ;; MOD1-NEXT:   (call_ref $0
- ;; MOD1-NEXT:    (ref.cast (ref (exact $0))
- ;; MOD1-NEXT:     (ref.func $trampoline_std::operator<<\28std::__2::basic_ostream<char\2c\20std::__2::char_traits<char>>&\2c\20wasm::Module&\29)
- ;; MOD1-NEXT:    )
+ ;; MOD1-NEXT:    (ref.func $trampoline_std::operator<<\28std::__2::basic_ostream<char\2c\20std::__2::char_traits<char>>&\2c\20wasm::Module&\29)
  ;; MOD1-NEXT:   )
  ;; MOD1-NEXT:  )
  ;; MOD1-NEXT:  (i32.const 0)
@@ -79,18 +75,16 @@
 
  ;; MOD2:      (import "primary" "table" (table $timport$0 3 funcref))
 
- ;; MOD2:      (import "primary" "trampoline_std::operator<<\\28std::__2::basic_ostream<char\\2c\\20std::__2::char_traits<char>>&\\2c\\20wasm::Module&\\29" (func $trampoline_std::operator<<\28std::__2::basic_ostream<char\2c\20std::__2::char_traits<char>>&\2c\20wasm::Module&\29 (result f32)))
+ ;; MOD2:      (import "primary" "trampoline_std::operator<<\\28std::__2::basic_ostream<char\\2c\\20std::__2::char_traits<char>>&\\2c\\20wasm::Module&\\29" (func $trampoline_std::operator<<\28std::__2::basic_ostream<char\2c\20std::__2::char_traits<char>>&\2c\20wasm::Module&\29 (exact (result f32))))
 
- ;; MOD2:      (import "primary" "trampoline_wasm::Type::getFeatures\\28\\29\\20const" (func $trampoline_wasm::Type::getFeatures\28\29\20const (result i32)))
+ ;; MOD2:      (import "primary" "trampoline_wasm::Type::getFeatures\\28\\29\\20const" (func $trampoline_wasm::Type::getFeatures\28\29\20const (exact (result i32))))
 
  ;; MOD2:      (elem $0 (i32.const 0) $wasm::Literal::Literal\28std::__2::array<wasm::Literal\2c\204ul>\20const&\29)
 
  ;; MOD2:      (func $wasm::Literal::Literal\28std::__2::array<wasm::Literal\2c\204ul>\20const&\29 (result i64)
  ;; MOD2-NEXT:  (drop
  ;; MOD2-NEXT:   (call_ref $1
- ;; MOD2-NEXT:    (ref.cast (ref (exact $1))
- ;; MOD2-NEXT:     (ref.func $trampoline_wasm::Type::getFeatures\28\29\20const)
- ;; MOD2-NEXT:    )
+ ;; MOD2-NEXT:    (ref.func $trampoline_wasm::Type::getFeatures\28\29\20const)
  ;; MOD2-NEXT:   )
  ;; MOD2-NEXT:  )
  ;; MOD2-NEXT:  (drop
@@ -100,9 +94,7 @@
  ;; MOD2-NEXT:  )
  ;; MOD2-NEXT:  (drop
  ;; MOD2-NEXT:   (call_ref $0
- ;; MOD2-NEXT:    (ref.cast (ref (exact $0))
- ;; MOD2-NEXT:     (ref.func $trampoline_std::operator<<\28std::__2::basic_ostream<char\2c\20std::__2::char_traits<char>>&\2c\20wasm::Module&\29)
- ;; MOD2-NEXT:    )
+ ;; MOD2-NEXT:    (ref.func $trampoline_std::operator<<\28std::__2::basic_ostream<char\2c\20std::__2::char_traits<char>>&\2c\20wasm::Module&\29)
  ;; MOD2-NEXT:   )
  ;; MOD2-NEXT:  )
  ;; MOD2-NEXT:  (i64.const 0)
@@ -134,25 +126,21 @@
 
  ;; MOD3:      (import "primary" "table" (table $timport$0 3 funcref))
 
- ;; MOD3:      (import "primary" "trampoline_wasm::Literal::Literal\\28std::__2::array<wasm::Literal\\2c\\204ul>\\20const&\\29" (func $trampoline_wasm::Literal::Literal\28std::__2::array<wasm::Literal\2c\204ul>\20const&\29 (result i64)))
+ ;; MOD3:      (import "primary" "trampoline_wasm::Literal::Literal\\28std::__2::array<wasm::Literal\\2c\\204ul>\\20const&\\29" (func $trampoline_wasm::Literal::Literal\28std::__2::array<wasm::Literal\2c\204ul>\20const&\29 (exact (result i64))))
 
- ;; MOD3:      (import "primary" "trampoline_wasm::Type::getFeatures\\28\\29\\20const" (func $trampoline_wasm::Type::getFeatures\28\29\20const (result i32)))
+ ;; MOD3:      (import "primary" "trampoline_wasm::Type::getFeatures\\28\\29\\20const" (func $trampoline_wasm::Type::getFeatures\28\29\20const (exact (result i32))))
 
  ;; MOD3:      (elem $0 (i32.const 1) $std::operator<<\28std::__2::basic_ostream<char\2c\20std::__2::char_traits<char>>&\2c\20wasm::Module&\29)
 
  ;; MOD3:      (func $std::operator<<\28std::__2::basic_ostream<char\2c\20std::__2::char_traits<char>>&\2c\20wasm::Module&\29 (result f32)
  ;; MOD3-NEXT:  (drop
  ;; MOD3-NEXT:   (call_ref $1
- ;; MOD3-NEXT:    (ref.cast (ref (exact $1))
- ;; MOD3-NEXT:     (ref.func $trampoline_wasm::Type::getFeatures\28\29\20const)
- ;; MOD3-NEXT:    )
+ ;; MOD3-NEXT:    (ref.func $trampoline_wasm::Type::getFeatures\28\29\20const)
  ;; MOD3-NEXT:   )
  ;; MOD3-NEXT:  )
  ;; MOD3-NEXT:  (drop
  ;; MOD3-NEXT:   (call_ref $0
- ;; MOD3-NEXT:    (ref.cast (ref (exact $0))
- ;; MOD3-NEXT:     (ref.func $trampoline_wasm::Literal::Literal\28std::__2::array<wasm::Literal\2c\204ul>\20const&\29)
- ;; MOD3-NEXT:    )
+ ;; MOD3-NEXT:    (ref.func $trampoline_wasm::Literal::Literal\28std::__2::array<wasm::Literal\2c\204ul>\20const&\29)
  ;; MOD3-NEXT:   )
  ;; MOD3-NEXT:  )
  ;; MOD3-NEXT:  (drop