[Optimize Instructions] Simplify sign extentions (Part 2) (#4382)

Similar to #4004 but for 32-bit integers

i32(x) << 24 >> 24   ==>   i32.extend8_s(x)
i32(x) << 16 >> 16   ==>   i32.extend16_s(x)

1 files changed, 16 insertions, 0 deletions

diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index c4a5b2ed4..c4ce8c239 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -476,6 +476,22 @@ struct OptimizeInstructions
                 break;
             }
           }
+          // i32(x) << 24 >> 24   ==>   i32.extend8_s(x)
+          // i32(x) << 16 >> 16   ==>   i32.extend16_s(x)
+          if (matches(curr,
+                      binary(ShrSInt32,
+                             binary(ShlInt32, any(&x), i32(&c1)),
+                             i32(&c2))) &&
+              Bits::getEffectiveShifts(c1) == Bits::getEffectiveShifts(c2)) {
+            switch (32 - Bits::getEffectiveShifts(c1)) {
+              case 8:
+                return replaceCurrent(builder.makeUnary(ExtendS8Int32, x));
+              case 16:
+                return replaceCurrent(builder.makeUnary(ExtendS16Int32, x));
+              default:
+                break;
+            }
+          }
         }
       }
       {