No need to subtract 1 before doing a right shift

roconnor-blockstream · web-flow · commit 3e43041be68c · 2023-03-27T09:29:41.000-04:00
diff --git a/src/int128_struct_impl.h b/src/int128_struct_impl.h
@@ -193,7 +193,7 @@ static SECP256K1_INLINE int secp256k1_i128_check_pow2(const secp256k1_int128 *r,
     VERIFY_CHECK(n < 127);
     VERIFY_CHECK(sign == 1 || sign == -1);
     return n >= 64 ? r->hi == (uint64_t)sign << (n - 64) && r->lo == 0
-                   : r->hi == (uint64_t)((sign - 1) >> 1) && r->lo == (uint64_t)sign << n;
+                   : r->hi == (uint64_t)(sign >> 1) && r->lo == (uint64_t)sign << n;
 }
 
 #endif

Original file line number	Diff line number	Diff line change
`@@ -193,7 +193,7 @@ static SECP256K1_INLINE int secp256k1_i128_check_pow2(const secp256k1_int128 *r,`
`193`	`193`	`VERIFY_CHECK(n < 127);`
`194`	`194`	`VERIFY_CHECK(sign == 1 \|\| sign == -1);`
`195`	`195`	`return n >= 64 ? r->hi == (uint64_t)sign << (n - 64) && r->lo == 0`
`196`		`- : r->hi == (uint64_t)((sign - 1) >> 1) && r->lo == (uint64_t)sign << n;`
	`196`	`+ : r->hi == (uint64_t)(sign >> 1) && r->lo == (uint64_t)sign << n;`
`197`	`197`	`}`
`198`	`198`
`199`	`199`	`#endif`