Fix PickLoadSigns on SignExt feature instructions (#7069)

I believe the history here is that

1.  We added a PickLoadSigns pass. It checks if a load from memory is stored in
    a local that is only every used in a signed or an unsigned manner. If it is, we can
    adjust the sign of the load (load8_u/s) to do the sign/unsign during the load.
2.  The pass finds each LocalGet and looks either 2 or 3 parents above it. For
    a sign operation, we need to look up 3, since the operation is x << K >> K. For
    an unsigned, we need only 2, since we have x & M. We hardcoded those
    numbers 2 and 3.
3.  We added the SignExt feature, which adds i32.extend8_s. This does a sign
    extend with a single instruction, not two nested ones, so now we can sign-
    extend at depth 2, unlike before. Properties::getSignExtValue was updated
    for this, but not the pass PickLoadSigns.

The bug that is fixed here is that we looked at depth 3 for a sign-extend, and
we blindly accepted it if we found one. So we ended up accepting
(i32.extend8_s (ANYTHING (x))), which is a sign-extend of something, but
not of x, which is bad.

We were also missing an optimization opportunity, as we didn't look for
depth 2 sign extends.

This bug is quite old, from when Properties got SignExt support, in #3910.
But the blame isn't there - to notice this then, we'd have had to check each
caller of getSignExtValue throughout the codebase, which isn't reasonable.
The fault is mine, from the first write-up of PickLoadSigns in 2017: the code
should have been fully general, handling 2/3 and checking the output when
it does so (adding == curr, that the sign/zero-extended value is the one we
expect). That is what this PR does.

1 files changed, 80 insertions, 0 deletions

diff --git a/test/lit/passes/pick-load-signs_sign-ext.wast b/test/lit/passes/pick-load-signs_sign-ext.wast
new file mode 100644
index 000000000..c50af32c5
--- /dev/null
+++ b/test/lit/passes/pick-load-signs_sign-ext.wast
@@ -0,0 +1,80 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
+;; RUN: wasm-opt %s --pick-load-signs -all -S -o - | filecheck %s
+
+(module
+ ;; CHECK:      (memory $0 16 17 shared)
+ (memory $0 16 17 shared)
+
+ ;; CHECK:      (func $load-other-use (type $0) (result i32)
+ ;; CHECK-NEXT:  (local $temp i32)
+ ;; CHECK-NEXT:  (block $label (result i32)
+ ;; CHECK-NEXT:   (local.set $temp
+ ;; CHECK-NEXT:    (i32.load8_u
+ ;; CHECK-NEXT:     (i32.const 22)
+ ;; CHECK-NEXT:    )
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:   (drop
+ ;; CHECK-NEXT:    (i32.extend8_s
+ ;; CHECK-NEXT:     (br_if $label
+ ;; CHECK-NEXT:      (local.get $temp)
+ ;; CHECK-NEXT:      (i32.const 1)
+ ;; CHECK-NEXT:     )
+ ;; CHECK-NEXT:    )
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:   (unreachable)
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT: )
+ (func $load-other-use (result i32)
+  (local $temp i32)
+  ;; The load here is unsigned, while the value in the local is used in two
+  ;; ways: it is sign-extended, and it is sent on a branch by a br_if. We must
+  ;; not ignore the branch, as if we did then we'd think the only use is signed,
+  ;; and we'd optimize load8_u => load8_s.
+  (block $label (result i32)
+   (local.set $temp
+    (i32.load8_u
+     (i32.const 22)
+    )
+   )
+   (drop
+    (i32.extend8_s
+     (br_if $label
+      (local.get $temp)
+      (i32.const 1)
+     )
+    )
+   )
+   (unreachable)
+  )
+ )
+
+ ;; CHECK:      (func $load-valid (type $1)
+ ;; CHECK-NEXT:  (local $temp i32)
+ ;; CHECK-NEXT:  (local.set $temp
+ ;; CHECK-NEXT:   (i32.load8_s
+ ;; CHECK-NEXT:    (i32.const 22)
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT:  (drop
+ ;; CHECK-NEXT:   (i32.extend8_s
+ ;; CHECK-NEXT:    (local.get $temp)
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT: )
+ (func $load-valid
+  (local $temp i32)
+  ;; As above, but remove the br_if in the middle. Now this is a valid case to
+  ;; optimize, load8_u => load8_s.
+  (local.set $temp
+   (i32.load8_u
+    (i32.const 22)
+   )
+  )
+  (drop
+   (i32.extend8_s
+    (local.get $temp)
+   )
+  )
+ )
+)
+