fails only on even 85 shifts

reilabs · Feb 8, 2025 · c64abd4 · c64abd4
1 parent 5c2acc7
commit c64abd4
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Showing 2 changed files with 16 additions and 17 deletions.
diff --git a/compiler-rt/src/alu/sshl_sat.cairo b/compiler-rt/src/alu/sshl_sat.cairo
@@ -1,6 +1,6 @@
 pub mod sshl_sat_i8;
 
-use crate::utils::{assert_fits_in_type, negate_twos_complement};
+use crate::utils::{assert_fits_in_type, negate_twos_complement, extend_sign};
 use crate::alu::shl::shl;
 use core::num::traits::{BitSize, Bounded};
 
@@ -54,31 +54,31 @@ fn sshl_sat<
     // Check if the shifted value is negative
     let sign_bit_mask = shl::<u128>(1, bit_size - 1);
     let shifted_sign_bit = (shifted & sign_bit_mask) != 0;
+    let n_sign_bit = (n & sign_bit_mask) != 0;
 
     // Min/max values of iN
     let max_value = sign_bit_mask - 1;
     let min_value = sign_bit_mask;
     #[cairofmt::skip]
-    let result = match (shifted > max_value, shifted_sign_bit) {
-        (true, _) => {
-            // The value after shifting exceeds the maximum value of the signed
-            // type. Clamp to the maximum possible value.
+    let result = match (n_sign_bit, shifted_sign_bit) {
+        (false, false) => {
+            println!("shifted {:x} extend_sign(shifted) {:x}, negate_twos_complement(shifted) {:x}, negate_twos_complement(extend_sign(shifted)) {:x}", shifted, extend_sign(shifted, sign_bit_mask), negate_twos_complement(shifted), negate_twos_complement(extend_sign(shifted, sign_bit_mask)));
+            shifted
+        },
+        (false, true) => {
             max_value
         },
-        (_, true) => {
-            // The value after shifting is negative. Since we're operating in
-            // u128, calculate compare its absolute value agains the minimum
-            // allowed value.
-            let shifted_abs = negate_twos_complement(shifted);
-            if shifted_abs > min_value {
-                // The value after shifting exceeds the minimum value of the
-                // signed type. Clamp to the minimum possible value.
-                min_value
-            } else {
+        (true, false) => {
+            min_value
+        },
+        (true, true) => {
+            println!("shifted {:x} extend_sign(shifted) {:x}, negate_twos_complement(shifted) {:x}, negate_twos_complement(extend_sign(shifted)) {:x}", shifted, extend_sign(shifted, sign_bit_mask), negate_twos_complement(shifted), negate_twos_complement(extend_sign(shifted, sign_bit_mask)));
+            if (extend_sign(shifted, shl::<u128>(1, bit_size - 1 + shift)) & ~Bounded::<T>::MAX.into()) == ~Bounded::<T>::MAX.into() {
                 shifted
+            } else {
+                min_value
             }
         },
-        _ => shifted,
     };
 
     result & Bounded::<T>::MAX.into()

diff --git a/compiler-rt/src/alu/sshl_sat/sshl_sat_i8.cairo b/compiler-rt/src/alu/sshl_sat/sshl_sat_i8.cairo
@@ -118,7 +118,6 @@ mod tests {
     #[should_panic(expected: "Requested shift by more bits than input word size")]
     fn test_i8_panic() {
         for case in test_cases_panic.span() {
-            println!("lhs {} rhs {} expected {}", *case.lhs, *case.rhs, *case.expected);
             assert_eq!(__llvm_sshl_sat_i8_i8(*case.lhs, *case.rhs), *case.expected);
         }
     }