Constant-fold icmp instructions (#5666)

We found examples of icmp instructions with both operands constant in
spidermonkey.wasm.

cranelift/codegen/src/isle_prelude.rs

@@ -134,6 +134,28 @@ macro_rules! isle_common_prelude_methods {
             x & 0xffff_ffff
         }
 
+        #[inline]
+        fn imm64_icmp(&mut self, ty: Type, cc: &IntCC, x: Imm64, y: Imm64) -> Imm64 {
+            let shift = u32::checked_sub(64, ty.bits()).unwrap_or(0);
+            let mask = u64::MAX >> shift;
+            let x = (x.bits() as u64) & mask;
+            let y = (y.bits() as u64) & mask;
+            let sext = |v| ((v << shift) as i64) >> shift;
+            let result = match cc {
+                IntCC::Equal => x == y,
+                IntCC::NotEqual => x != y,
+                IntCC::UnsignedGreaterThanOrEqual => x >= y,
+                IntCC::UnsignedGreaterThan => x > y,
+                IntCC::UnsignedLessThanOrEqual => x <= y,
+                IntCC::UnsignedLessThan => x < y,
+                IntCC::SignedGreaterThanOrEqual => sext(x) >= sext(y),
+                IntCC::SignedGreaterThan => sext(x) > sext(y),
+                IntCC::SignedLessThanOrEqual => sext(x) <= sext(y),
+                IntCC::SignedLessThan => sext(x) < sext(y),
+            };
+            Imm64::new(result.into())
+        }
+
         #[inline]
         fn ty_bits(&mut self, ty: Type) -> u8 {
             use std::convert::TryInto;

cranelift/codegen/src/opts/cprop.isle

@@ -70,6 +70,14 @@
                       (iconst ty k2)))
       (subsume (iconst ty (imm64_sshr ty k1 k2))))
 
+(rule (simplify
+       (icmp result_ty
+            cc
+            (iconst ty k1)
+            (iconst ty k2)))
+      (subsume (iconst result_ty (imm64_icmp ty cc k1 k2))))
+
+
 ;; Canonicalize via commutativity: push immediates to the right.
 ;;
 ;;   (op k x) --> (op x k)

cranelift/codegen/src/prelude.isle

@@ -150,6 +150,9 @@
 (decl pure u64_uextend_u32 (u64) u64)
 (extern constructor u64_uextend_u32 u64_uextend_u32)
 
+(decl pure imm64_icmp (Type IntCC Imm64 Imm64) Imm64)
+(extern constructor imm64_icmp imm64_icmp)
+
 (decl u64_is_zero (bool) u64)
 (extern extractor infallible u64_is_zero u64_is_zero)