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_32.c b/p521_32.c
index d852bb285..5e2fd8a15 100644
--- a/p521_32.c
+++ b/p521_32.c
@@ -41,7 +41,7 @@ static void fiat_p521_addcarryx_u30(uint32_t* out1, fiat_p521_uint1* out2, fiat_
  */
 static void fiat_p521_subborrowx_u30(uint32_t* out1, fiat_p521_uint1* out2, fiat_p521_uint1 arg1, uint32_t arg2, uint32_t arg3) {
   int32_t x1 = ((int32_t)(arg2 - arg1) - (int32_t)arg3);
-  fiat_p521_int1 x2 = (fiat_p521_int1)((uint32_t)x1 >> 30);
+  fiat_p521_int1 x2 = (fiat_p521_int1)((int64_t)x1 >> 30);
   uint32_t x3 = (x1 & UINT32_C(0x3fffffff));
   *out1 = x3;
   *out2 = (fiat_p521_uint1)(0x0 - x2);
@@ -75,7 +75,7 @@ static void fiat_p521_addcarryx_u31(uint32_t* out1, fiat_p521_uint1* out2, fiat_
  */
 static void fiat_p521_subborrowx_u31(uint32_t* out1, fiat_p521_uint1* out2, fiat_p521_uint1 arg1, uint32_t arg2, uint32_t arg3) {
   int32_t x1 = ((int32_t)(arg2 - arg1) - (int32_t)arg3);
-  fiat_p521_int1 x2 = (fiat_p521_int1)((uint32_t)x1 >> 31);
+  fiat_p521_int1 x2 = (fiat_p521_int1)((int64_t)x1 >> 31);
   uint32_t x3 = (x1 & UINT32_C(0x7fffffff));
   *out1 = x3;
   *out2 = (fiat_p521_uint1)(0x0 - x2);
@@ -1200,7 +1200,7 @@ static void fiat_p521_to_bytes(uint8_t out1[66], const uint32_t arg1[17]) {
   uint8_t x130 = (uint8_t)(x127 & UINT8_C(0xff));
   uint8_t x131 = (uint8_t)(x129 >> 8);
   uint8_t x132 = (uint8_t)(x129 & UINT8_C(0xff));
-  fiat_p521_uint1 x133 = (fiat_p521_uint1)(x131 >> 8);
+  fiat_p521_uint1 x133 = (fiat_p521_uint1)((int64_t)x131 >> 8);
   uint8_t x134 = (uint8_t)(x131 & UINT8_C(0xff));
   uint32_t x135 = (x133 + x48);
   uint32_t x136 = (x135 >> 8);
@@ -1252,7 +1252,7 @@ static void fiat_p521_to_bytes(uint8_t out1[66], const uint32_t arg1[17]) {
   uint8_t x182 = (uint8_t)(x179 & UINT8_C(0xff));
   uint8_t x183 = (uint8_t)(x181 >> 8);
   uint8_t x184 = (uint8_t)(x181 & UINT8_C(0xff));
-  fiat_p521_uint1 x185 = (fiat_p521_uint1)(x183 >> 8);
+  fiat_p521_uint1 x185 = (fiat_p521_uint1)((int64_t)x183 >> 8);
   uint8_t x186 = (uint8_t)(x183 & UINT8_C(0xff));
   uint32_t x187 = (x185 + x60);
   uint32_t x188 = (x187 >> 8);