egraph opts: fix uextend-of-i32. (#5061)

This is a simple error in the const-prop rules: uextend was not
masking iconst's u64 immediate when extending from i32 to
i64. Arguably an iconst.i32 should not have nonzero bits in the upper
32 of its immediate, but that's a separate design question. For now,
if our invariant is that the upper bits are ignored, then it is
required to mask the bits when const-evaling a `uextend`.

Fixes #5047.

cranelift/codegen/src/isle_prelude.rs

@@ -95,6 +95,11 @@ macro_rules! isle_common_prelude_methods {
             Some(x as u32 as i32 as i64 as u64)
         }
 
+        #[inline]
+        fn u64_uextend_u32(&mut self, x: u64) -> Option<u64> {
+            Some(x & 0xffff_ffff)
+        }
+
         #[inline]
         fn ty_bits(&mut self, ty: Type) -> Option<u8> {
             use std::convert::TryInto;

cranelift/codegen/src/opts/algebraic.isle

@@ -7,8 +7,8 @@
 ;; can freely rewrite e.g. `x+y-y` to `x`.
 
 ;; uextend/sextend of a constant.
-(rule (simplify (uextend $I64 (iconst $I32 imm)))
-      (iconst $I64 imm))
+(rule (simplify (uextend $I64 (iconst $I32 (u64_from_imm64 imm))))
+      (iconst $I64 (imm64 (u64_uextend_u32 imm))))
 (rule (simplify (sextend $I64 (iconst $I32 (u64_from_imm64 imm))))
       (iconst $I64 (imm64 (u64_sextend_u32 imm))))
 

cranelift/codegen/src/prelude.isle

@@ -125,6 +125,9 @@
 (decl pure u64_sextend_u32 (u64) u64)
 (extern constructor u64_sextend_u32 u64_sextend_u32)
 
+(decl pure u64_uextend_u32 (u64) u64)
+(extern constructor u64_uextend_u32 u64_uextend_u32)
+
 (decl u64_is_zero (bool) u64)
 (extern extractor infallible u64_is_zero u64_is_zero)