Improve constant-timeness on PowerPC

Summary:
 * Remove redundant "? 1 : 0" after comparisons in scalar code

This prevents GCC from generating branches on PowerPC in certain
cases.

Fixes #771.

 * Suppress a harmless variable-time optimization by clang in _int_cmov

Follow up on 52a03512c1d800603b5c923c1a28bdba12dadb30

This is a backport of libsecp256k1 [[bitcoin-core/secp256k1#772 | PR772]]

Depends on D7590

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7597

Author: real-or-random

src/secp256k1/src/scalar_4x64_impl.h

@@ -192,9 +192,9 @@ static int secp256k1_scalar_cond_negate(secp256k1_scalar *r, int flag) {
         tl = t; \
     } \
     c0 += tl;                 /* overflow is handled on the next line */ \
-    th += (c0 < tl) ? 1 : 0;  /* at most 0xFFFFFFFFFFFFFFFF */ \
+    th += (c0 < tl);          /* at most 0xFFFFFFFFFFFFFFFF */ \
     c1 += th;                 /* overflow is handled on the next line */ \
-    c2 += (c1 < th) ? 1 : 0;  /* never overflows by contract (verified in the next line) */ \
+    c2 += (c1 < th);          /* never overflows by contract (verified in the next line) */ \
     VERIFY_CHECK((c1 >= th) || (c2 != 0)); \
 }
 
@@ -207,7 +207,7 @@ static int secp256k1_scalar_cond_negate(secp256k1_scalar *r, int flag) {
         tl = t; \
     } \
     c0 += tl;                 /* overflow is handled on the next line */ \
-    th += (c0 < tl) ? 1 : 0;  /* at most 0xFFFFFFFFFFFFFFFF */ \
+    th += (c0 < tl);          /* at most 0xFFFFFFFFFFFFFFFF */ \
     c1 += th;                 /* never overflows by contract (verified in the next line) */ \
     VERIFY_CHECK(c1 >= th); \
 }
@@ -221,32 +221,32 @@ static int secp256k1_scalar_cond_negate(secp256k1_scalar *r, int flag) {
         tl = t; \
     } \
     th2 = th + th;                  /* at most 0xFFFFFFFFFFFFFFFE (in case th was 0x7FFFFFFFFFFFFFFF) */ \
-    c2 += (th2 < th) ? 1 : 0;       /* never overflows by contract (verified the next line) */ \
+    c2 += (th2 < th);               /* never overflows by contract (verified the next line) */ \
     VERIFY_CHECK((th2 >= th) || (c2 != 0)); \
     tl2 = tl + tl;                  /* at most 0xFFFFFFFFFFFFFFFE (in case the lowest 63 bits of tl were 0x7FFFFFFFFFFFFFFF) */ \
-    th2 += (tl2 < tl) ? 1 : 0;      /* at most 0xFFFFFFFFFFFFFFFF */ \
+    th2 += (tl2 < tl);              /* at most 0xFFFFFFFFFFFFFFFF */ \
     c0 += tl2;                      /* overflow is handled on the next line */ \
-    th2 += (c0 < tl2) ? 1 : 0;      /* second overflow is handled on the next line */ \
+    th2 += (c0 < tl2);              /* second overflow is handled on the next line */ \
     c2 += (c0 < tl2) & (th2 == 0);  /* never overflows by contract (verified the next line) */ \
     VERIFY_CHECK((c0 >= tl2) || (th2 != 0) || (c2 != 0)); \
     c1 += th2;                      /* overflow is handled on the next line */ \
-    c2 += (c1 < th2) ? 1 : 0;       /* never overflows by contract (verified the next line) */ \
+    c2 += (c1 < th2);               /* never overflows by contract (verified the next line) */ \
     VERIFY_CHECK((c1 >= th2) || (c2 != 0)); \
 }
 
 /** Add a to the number defined by (c0,c1,c2). c2 must never overflow. */
 #define sumadd(a) { \
     unsigned int over; \
     c0 += (a);                  /* overflow is handled on the next line */ \
-    over = (c0 < (a)) ? 1 : 0; \
+    over = (c0 < (a));         \
     c1 += over;                 /* overflow is handled on the next line */ \
-    c2 += (c1 < over) ? 1 : 0;  /* never overflows by contract */ \
+    c2 += (c1 < over);          /* never overflows by contract */ \
 }
 
 /** Add a to the number defined by (c0,c1). c1 must never overflow, c2 must be zero. */
 #define sumadd_fast(a) { \
     c0 += (a);                 /* overflow is handled on the next line */ \
-    c1 += (c0 < (a)) ? 1 : 0;  /* never overflows by contract (verified the next line) */ \
+    c1 += (c0 < (a));          /* never overflows by contract (verified the next line) */ \
     VERIFY_CHECK((c1 != 0) | (c0 >= (a))); \
     VERIFY_CHECK(c2 == 0); \
 }

src/secp256k1/src/scalar_8x32_impl.h

@@ -271,9 +271,9 @@ static int secp256k1_scalar_cond_negate(secp256k1_scalar *r, int flag) {
         tl = t; \
     } \
     c0 += tl;                 /* overflow is handled on the next line */ \
-    th += (c0 < tl) ? 1 : 0;  /* at most 0xFFFFFFFF */ \
+    th += (c0 < tl);          /* at most 0xFFFFFFFF */ \
     c1 += th;                 /* overflow is handled on the next line */ \
-    c2 += (c1 < th) ? 1 : 0;  /* never overflows by contract (verified in the next line) */ \
+    c2 += (c1 < th);          /* never overflows by contract (verified in the next line) */ \
     VERIFY_CHECK((c1 >= th) || (c2 != 0)); \
 }
 
@@ -286,7 +286,7 @@ static int secp256k1_scalar_cond_negate(secp256k1_scalar *r, int flag) {
         tl = t; \
     } \
     c0 += tl;                 /* overflow is handled on the next line */ \
-    th += (c0 < tl) ? 1 : 0;  /* at most 0xFFFFFFFF */ \
+    th += (c0 < tl);          /* at most 0xFFFFFFFF */ \
     c1 += th;                 /* never overflows by contract (verified in the next line) */ \
     VERIFY_CHECK(c1 >= th); \
 }
@@ -300,32 +300,32 @@ static int secp256k1_scalar_cond_negate(secp256k1_scalar *r, int flag) {
         tl = t; \
     } \
     th2 = th + th;                  /* at most 0xFFFFFFFE (in case th was 0x7FFFFFFF) */ \
-    c2 += (th2 < th) ? 1 : 0;       /* never overflows by contract (verified the next line) */ \
+    c2 += (th2 < th);               /* never overflows by contract (verified the next line) */ \
     VERIFY_CHECK((th2 >= th) || (c2 != 0)); \
     tl2 = tl + tl;                  /* at most 0xFFFFFFFE (in case the lowest 63 bits of tl were 0x7FFFFFFF) */ \
-    th2 += (tl2 < tl) ? 1 : 0;      /* at most 0xFFFFFFFF */ \
+    th2 += (tl2 < tl);              /* at most 0xFFFFFFFF */ \
     c0 += tl2;                      /* overflow is handled on the next line */ \
-    th2 += (c0 < tl2) ? 1 : 0;      /* second overflow is handled on the next line */ \
+    th2 += (c0 < tl2);              /* second overflow is handled on the next line */ \
     c2 += (c0 < tl2) & (th2 == 0);  /* never overflows by contract (verified the next line) */ \
     VERIFY_CHECK((c0 >= tl2) || (th2 != 0) || (c2 != 0)); \
     c1 += th2;                      /* overflow is handled on the next line */ \
-    c2 += (c1 < th2) ? 1 : 0;       /* never overflows by contract (verified the next line) */ \
+    c2 += (c1 < th2);               /* never overflows by contract (verified the next line) */ \
     VERIFY_CHECK((c1 >= th2) || (c2 != 0)); \
 }
 
 /** Add a to the number defined by (c0,c1,c2). c2 must never overflow. */
 #define sumadd(a) { \
     unsigned int over; \
     c0 += (a);                  /* overflow is handled on the next line */ \
-    over = (c0 < (a)) ? 1 : 0; \
+    over = (c0 < (a)); \
     c1 += over;                 /* overflow is handled on the next line */ \
-    c2 += (c1 < over) ? 1 : 0;  /* never overflows by contract */ \
+    c2 += (c1 < over);          /* never overflows by contract */ \
 }
 
 /** Add a to the number defined by (c0,c1). c1 must never overflow, c2 must be zero. */
 #define sumadd_fast(a) { \
     c0 += (a);                 /* overflow is handled on the next line */ \
-    c1 += (c0 < (a)) ? 1 : 0;  /* never overflows by contract (verified the next line) */ \
+    c1 += (c0 < (a));          /* never overflows by contract (verified the next line) */ \
     VERIFY_CHECK((c1 != 0) | (c0 >= (a))); \
     VERIFY_CHECK(c2 == 0); \
 }

src/secp256k1/src/util.h

@@ -197,10 +197,15 @@ static SECP256K1_INLINE void memczero(void *s, size_t len, int flag) {
 /** If flag is true, set *r equal to *a; otherwise leave it. Constant-time.  Both *r and *a must be initialized and non-negative.*/
 static SECP256K1_INLINE void secp256k1_int_cmov(int *r, const int *a, int flag) {
     unsigned int mask0, mask1, r_masked, a_masked;
+    /* Access flag with a volatile-qualified lvalue.
+       This prevents clang from figuring out (after inlining) that flag can
+       take only be 0 or 1, which leads to variable time code. */
+    volatile int vflag = flag;
+
     /* Casting a negative int to unsigned and back to int is implementation defined behavior */
     VERIFY_CHECK(*r >= 0 && *a >= 0);
 
-    mask0 = (unsigned int)flag + ~0u;
+    mask0 = (unsigned int)vflag + ~0u;
     mask1 = ~mask0;
     r_masked = ((unsigned int)*r & mask0);
     a_masked = ((unsigned int)*a & mask1);