StringGathering pass (#6257)

This pass finds all string.const and creates globals for them. After this transform, no
string.const appears anywhere but in a global, and each string appears in one global
which is then global.get-ed everywhere.

This avoids overhead in VMs where executing a string.const is an allocation, and is
also a good step towards imported strings. For that, this pass will be extended from
gathering to a full lowering pass, which will first gather into globals as this pass does,
and then turn each of those globals with a string.const into an imported externref.
(For that reason this pass is in a file called StringLowering, as the two passes will
share much of their code, and the larger pass should decide the name I think.)

This pass runs in -O2 and above. Repeated executions have no downside (see
details in code).

1 files changed, 96 insertions, 0 deletions

diff --git a/test/lit/passes/string-gathering.wast b/test/lit/passes/string-gathering.wast
new file mode 100644
index 000000000..21fe358a3
--- /dev/null
+++ b/test/lit/passes/string-gathering.wast
@@ -0,0 +1,96 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py --all-items and should not be edited.
+
+;; RUN: foreach %s %t wasm-opt --string-gathering -all -S -o - | filecheck %s
+
+;; All the strings should be collected into globals and used from there. They
+;; should also be sorted deterministically (alphabetically).
+
+(module
+  ;; Note that $global will be reused: no new global will be added for "foo".
+  ;; $global2 almost can, but it has the wrong type, so it won't.
+
+  ;; CHECK:      (type $0 (func))
+
+  ;; CHECK:      (global $string.const_bar (ref string) (string.const "bar"))
+
+  ;; CHECK:      (global $string.const_other (ref string) (string.const "other"))
+
+  ;; CHECK:      (global $global (ref string) (string.const "foo"))
+  (global $global (ref string) (string.const "foo"))
+
+  ;; CHECK:      (global $global2 stringref (global.get $string.const_bar))
+  (global $global2 (ref null string) (string.const "bar"))
+
+  ;; CHECK:      (func $a (type $0)
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (global.get $string.const_bar)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (global.get $global)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  (func $a
+    (drop
+      (string.const "bar")
+    )
+    (drop
+      (string.const "foo")
+    )
+  )
+
+  ;; CHECK:      (func $b (type $0)
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (global.get $string.const_bar)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (global.get $string.const_other)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (global.get $global)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT:  (drop
+  ;; CHECK-NEXT:   (global.get $global2)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  (func $b
+    (drop
+      (string.const "bar")
+    )
+    (drop
+      (string.const "other")
+    )
+    ;; Existing global.gets are not modified (but after this pass,
+    ;; SimplifyGlobals could help; though in practice the globals would have
+    ;; been propagated here anyhow).
+    (drop
+      (global.get $global)
+    )
+    (drop
+      (global.get $global2)
+    )
+  )
+)
+
+;; Multiple possible reusable globals. Also test ignoring of imports.
+(module
+  ;; CHECK:      (import "a" "b" (global $import (ref string)))
+  (import "a" "b" (global $import (ref string)))
+
+  ;; CHECK:      (global $global1 (ref string) (string.const "foo"))
+  (global $global1 (ref string) (string.const "foo"))
+
+  ;; CHECK:      (global $global2 (ref string) (global.get $global1))
+  (global $global2 (ref string) (string.const "foo"))
+
+  ;; CHECK:      (global $global3 (ref string) (global.get $global1))
+  (global $global3 (ref string) (string.const "foo"))
+
+  ;; CHECK:      (global $global4 (ref string) (string.const "bar"))
+  (global $global4 (ref string) (string.const "bar"))
+
+  ;; CHECK:      (global $global5 (ref string) (global.get $global4))
+  (global $global5 (ref string) (string.const "bar"))
+
+  ;; CHECK:      (global $global6 (ref string) (global.get $global4))
+  (global $global6 (ref string) (string.const "bar"))
+)