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, 0 insertions, 65 deletions

diff --git a/test/lit/passes/simplify-globals-strings.wast b/test/lit/passes/simplify-globals-strings.wast
deleted file mode 100644
index 813e2964c..000000000
--- a/test/lit/passes/simplify-globals-strings.wast
+++ /dev/null
@@ -1,65 +0,0 @@
-;; NOTE: Assertions have been generated by update_lit_checks.py --all-items and should not be edited.
-;; NOTE: This test was ported using port_passes_tests_to_lit.py and could be cleaned up.
-
-;; We do not "inline" strings from globals, as that might cause more
-;; allocations to happen. TODO if VMs optimize that, remove this
-
-;; RUN: foreach %s %t wasm-opt --simplify-globals -all -S -o - | filecheck %s
-
-;; Also test with -O3 --gufa to see that no other pass does this kind of thing,
-;; as extra coverage.
-
-;; RUN: foreach %s %t wasm-opt -O3 --gufa         -all -S -o - | filecheck %s --check-prefix=O3GUF
-
-(module
-  ;; CHECK:      (type $0 (func (result anyref)))
-
-  ;; CHECK:      (global $string stringref (string.const "one"))
-  ;; O3GUF:      (type $0 (func (result anyref)))
-
-  ;; O3GUF:      (global $string (ref string) (string.const "one"))
-  (global $string stringref (string.const "one"))
-
-  ;; CHECK:      (global $string-mut (mut stringref) (string.const "two"))
-  ;; O3GUF:      (global $string-mut (mut (ref string)) (string.const "two"))
-  (global $string-mut (mut stringref) (string.const "two"))
-
-  ;; CHECK:      (export "global" (func $global))
-  ;; O3GUF:      (export "global" (func $global))
-  (export "global" (func $global))
-
-  ;; CHECK:      (export "written" (func $written))
-  ;; O3GUF:      (export "written" (func $written))
-  (export "written" (func $written))
-
-  ;; CHECK:      (func $global (type $0) (result anyref)
-  ;; CHECK-NEXT:  (global.get $string)
-  ;; CHECK-NEXT: )
-  ;; O3GUF:      (func $global (type $0) (result anyref)
-  ;; O3GUF-NEXT:  (global.get $string)
-  ;; O3GUF-NEXT: )
-  (func $global (result anyref)
-    ;; This should not turn into "one".
-    (global.get $string)
-  )
-
-  ;; CHECK:      (func $written (type $0) (result anyref)
-  ;; CHECK-NEXT:  (global.set $string-mut
-  ;; CHECK-NEXT:   (string.const "three")
-  ;; CHECK-NEXT:  )
-  ;; CHECK-NEXT:  (global.get $string-mut)
-  ;; CHECK-NEXT: )
-  ;; O3GUF:      (func $written (type $0) (result anyref)
-  ;; O3GUF-NEXT:  (global.set $string-mut
-  ;; O3GUF-NEXT:   (string.const "three")
-  ;; O3GUF-NEXT:  )
-  ;; O3GUF-NEXT:  (global.get $string-mut)
-  ;; O3GUF-NEXT: )
-  (func $written (result anyref)
-    (global.set $string-mut
-      (string.const "three")
-    )
-    ;; This should not turn into "three".
-    (global.get $string-mut)
-  )
-)