Don't cast signed to unsigned before shifting

Unfortunately, signed->unsigned casts do not commute with shifts.  We
take care to only extend the range when it needs extending, now.  This
was previously causing issues with subborrow.

We should really get proofs about casts in C semantics at some point
soon.

Fixes #489

1 files changed, 4 insertions, 4 deletions

diff --git a/p521_64.c b/p521_64.c
index e8afff411..386f6241e 100644
--- a/p521_64.c
+++ b/p521_64.c
@@ -41,7 +41,7 @@ static void fiat_p521_addcarryx_u58(uint64_t* out1, fiat_p521_uint1* out2, fiat_
  */
 static void fiat_p521_subborrowx_u58(uint64_t* out1, fiat_p521_uint1* out2, fiat_p521_uint1 arg1, uint64_t arg2, uint64_t arg3) {
   int64_t x1 = ((int64_t)(arg2 - (int64_t)arg1) - (int64_t)arg3);
-  fiat_p521_int1 x2 = (fiat_p521_int1)((uint64_t)x1 >> 58);
+  fiat_p521_int1 x2 = (fiat_p521_int1)((fiat_p521_int128)x1 >> 58);
   uint64_t x3 = (x1 & UINT64_C(0x3ffffffffffffff));
   *out1 = x3;
   *out2 = (fiat_p521_uint1)(0x0 - x2);
@@ -75,7 +75,7 @@ static void fiat_p521_addcarryx_u57(uint64_t* out1, fiat_p521_uint1* out2, fiat_
  */
 static void fiat_p521_subborrowx_u57(uint64_t* out1, fiat_p521_uint1* out2, fiat_p521_uint1 arg1, uint64_t arg2, uint64_t arg3) {
   int64_t x1 = ((int64_t)(arg2 - (int64_t)arg1) - (int64_t)arg3);
-  fiat_p521_int1 x2 = (fiat_p521_int1)((uint64_t)x1 >> 57);
+  fiat_p521_int1 x2 = (fiat_p521_int1)((fiat_p521_int128)x1 >> 57);
   uint64_t x3 = (x1 & UINT64_C(0x1ffffffffffffff));
   *out1 = x3;
   *out2 = (fiat_p521_uint1)(0x0 - x2);
@@ -646,7 +646,7 @@ static void fiat_p521_to_bytes(uint8_t out1[66], const uint64_t arg1[9]) {
   uint8_t x100 = (uint8_t)(x97 & UINT8_C(0xff));
   uint8_t x101 = (uint8_t)(x99 >> 8);
   uint8_t x102 = (uint8_t)(x99 & UINT8_C(0xff));
-  fiat_p521_uint1 x103 = (fiat_p521_uint1)(x101 >> 8);
+  fiat_p521_uint1 x103 = (fiat_p521_uint1)((int64_t)x101 >> 8);
   uint8_t x104 = (uint8_t)(x101 & UINT8_C(0xff));
   uint64_t x105 = (x103 + x28);
   uint64_t x106 = (x105 >> 8);
@@ -708,7 +708,7 @@ static void fiat_p521_to_bytes(uint8_t out1[66], const uint64_t arg1[9]) {
   uint8_t x162 = (uint8_t)(x159 & UINT8_C(0xff));
   uint8_t x163 = (uint8_t)(x161 >> 8);
   uint8_t x164 = (uint8_t)(x161 & UINT8_C(0xff));
-  fiat_p521_uint1 x165 = (fiat_p521_uint1)(x163 >> 8);
+  fiat_p521_uint1 x165 = (fiat_p521_uint1)((int64_t)x163 >> 8);
   uint8_t x166 = (uint8_t)(x163 & UINT8_C(0xff));
   uint64_t x167 = (x165 + x36);
   uint64_t x168 = (x167 >> 8);