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, 2 insertions, 0 deletions

diff --git a/test/lit/help/wasm2js.test b/test/lit/help/wasm2js.test
index 7db564101..40d608857 100644
--- a/test/lit/help/wasm2js.test
+++ b/test/lit/help/wasm2js.test
@@ -426,6 +426,8 @@
 ;; CHECK-NEXT:   --stack-check                                 enforce limits on llvm's
 ;; CHECK-NEXT:                                                 __stack_pointer global
 ;; CHECK-NEXT:
+;; CHECK-NEXT:   --string-gathering                            gathers wasm strings to globals
+;; CHECK-NEXT:
 ;; CHECK-NEXT:   --strip                                       deprecated; same as strip-debug
 ;; CHECK-NEXT:
 ;; CHECK-NEXT:   --strip-debug                                 strip debug info (including the