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, 6 insertions, 6 deletions

diff --git a/p384_64.c b/p384_64.c
index c82ab721c..b4aff4a1e 100644
--- a/p384_64.c
+++ b/p384_64.c
@@ -46,7 +46,7 @@ static void fiat_p384_addcarryx_u64(uint64_t* out1, fiat_p384_uint1* out2, fiat_
  */
 static void fiat_p384_subborrowx_u64(uint64_t* out1, fiat_p384_uint1* out2, fiat_p384_uint1 arg1, uint64_t arg2, uint64_t arg3) {
   fiat_p384_int128 x1 = ((arg2 - (fiat_p384_int128)arg1) - arg3);
-  fiat_p384_int1 x2 = (fiat_p384_int1)((fiat_p384_uint128)x1 >> 64);
+  fiat_p384_int1 x2 = (fiat_p384_int1)((fiat_p384_int256)x1 >> 64);
   uint64_t x3 = (uint64_t)(x1 & UINT64_C(0xffffffffffffffff));
   *out1 = x3;
   *out2 = (fiat_p384_uint1)(0x0 - x2);
@@ -2363,7 +2363,7 @@ static void fiat_p384_to_bytes(uint8_t out1[48], const uint64_t arg1[6]) {
   uint8_t x18 = (uint8_t)(x15 & UINT8_C(0xff));
   uint8_t x19 = (uint8_t)(x17 >> 8);
   uint8_t x20 = (uint8_t)(x17 & UINT8_C(0xff));
-  fiat_p384_uint1 x21 = (fiat_p384_uint1)(x19 >> 8);
+  fiat_p384_uint1 x21 = (fiat_p384_uint1)((int64_t)x19 >> 8);
   uint8_t x22 = (uint8_t)(x19 & UINT8_C(0xff));
   uint64_t x23 = (x21 + x5);
   uint64_t x24 = (x23 >> 8);
@@ -2380,7 +2380,7 @@ static void fiat_p384_to_bytes(uint8_t out1[48], const uint64_t arg1[6]) {
   uint8_t x35 = (uint8_t)(x32 & UINT8_C(0xff));
   uint8_t x36 = (uint8_t)(x34 >> 8);
   uint8_t x37 = (uint8_t)(x34 & UINT8_C(0xff));
-  fiat_p384_uint1 x38 = (fiat_p384_uint1)(x36 >> 8);
+  fiat_p384_uint1 x38 = (fiat_p384_uint1)((int64_t)x36 >> 8);
   uint8_t x39 = (uint8_t)(x36 & UINT8_C(0xff));
   uint64_t x40 = (x38 + x4);
   uint64_t x41 = (x40 >> 8);
@@ -2397,7 +2397,7 @@ static void fiat_p384_to_bytes(uint8_t out1[48], const uint64_t arg1[6]) {
   uint8_t x52 = (uint8_t)(x49 & UINT8_C(0xff));
   uint8_t x53 = (uint8_t)(x51 >> 8);
   uint8_t x54 = (uint8_t)(x51 & UINT8_C(0xff));
-  fiat_p384_uint1 x55 = (fiat_p384_uint1)(x53 >> 8);
+  fiat_p384_uint1 x55 = (fiat_p384_uint1)((int64_t)x53 >> 8);
   uint8_t x56 = (uint8_t)(x53 & UINT8_C(0xff));
   uint64_t x57 = (x55 + x3);
   uint64_t x58 = (x57 >> 8);
@@ -2414,7 +2414,7 @@ static void fiat_p384_to_bytes(uint8_t out1[48], const uint64_t arg1[6]) {
   uint8_t x69 = (uint8_t)(x66 & UINT8_C(0xff));
   uint8_t x70 = (uint8_t)(x68 >> 8);
   uint8_t x71 = (uint8_t)(x68 & UINT8_C(0xff));
-  fiat_p384_uint1 x72 = (fiat_p384_uint1)(x70 >> 8);
+  fiat_p384_uint1 x72 = (fiat_p384_uint1)((int64_t)x70 >> 8);
   uint8_t x73 = (uint8_t)(x70 & UINT8_C(0xff));
   uint64_t x74 = (x72 + x2);
   uint64_t x75 = (x74 >> 8);
@@ -2431,7 +2431,7 @@ static void fiat_p384_to_bytes(uint8_t out1[48], const uint64_t arg1[6]) {
   uint8_t x86 = (uint8_t)(x83 & UINT8_C(0xff));
   uint8_t x87 = (uint8_t)(x85 >> 8);
   uint8_t x88 = (uint8_t)(x85 & UINT8_C(0xff));
-  fiat_p384_uint1 x89 = (fiat_p384_uint1)(x87 >> 8);
+  fiat_p384_uint1 x89 = (fiat_p384_uint1)((int64_t)x87 >> 8);
   uint8_t x90 = (uint8_t)(x87 & UINT8_C(0xff));
   uint64_t x91 = (x89 + x1);
   uint64_t x92 = (x91 >> 8);