fixup! feat(const_eval): impl. firstLeadingBit

Author: ErichDonGubler

naga/src/proc/constant_evaluator.rs

@@ -2188,31 +2188,23 @@ fn first_leading_bit(concrete_int: ConcreteInt<1>) -> ConcreteInt<1> {
     // NOTE: Bit indices for this built-in start at 0 at the "right" (or LSB). For example, 1 means
     // the least significant bit is set. Therefore, an input of 1 would return a right-to-left bit
     // index of 0.
-    let highest_rtl_bit_index = 32 /* bit */ - 1 /* count to zero index */;
+    let rtl_to_ltr_bit_idx = |e: u32| -> u32 {
+        match e {
+            idx @ 0..=31 => 31 - idx,
+            32 => u32::MAX,
+            _ => unreachable!(),
+        }
+    };
     match concrete_int {
         ConcreteInt::I32([e]) => ConcreteInt::I32([{
-            if matches!(e, 0 | -1) {
-                -1
-            } else {
-                let rtl_bit_index = if e.is_positive() {
-                    e.leading_zeros()
-                } else {
-                    // must be negative
-                    e.leading_ones()
-                };
-                (highest_rtl_bit_index - rtl_bit_index)
-                    .try_into()
-                    .expect("bit count outside of 0..32 (!?)")
-            }
-        }]),
-        ConcreteInt::U32([e]) => ConcreteInt::U32([{
-            if e == 0 {
-                u32::MAX
+            let rtl_bit_index = if e.is_negative() {
+                e.leading_ones()
             } else {
-                let rtl_bit_index = e.leading_zeros();
-                highest_rtl_bit_index - rtl_bit_index
-            }
+                e.leading_zeros()
+            };
+            rtl_to_ltr_bit_idx(rtl_bit_index) as i32
         }]),
+        ConcreteInt::U32([e]) => ConcreteInt::U32([rtl_to_ltr_bit_idx(e.leading_zeros())]),
     }
 }