Expand bit to mask as inline function

use the -1 to unsigned integer trick, disable windows C4146

5 files changed, 23 insertions, 7 deletions

diff --git a/src/include/word.h b/src/include/word.h
index d10a804..7eb4f2a 100644
--- a/src/include/word.h
+++ b/src/include/word.h
@@ -67,6 +67,22 @@ extern int posix_memalign(void **, size_t, size_t);
 #else
     #error "For now, libdecaf only supports 32- and 64-bit architectures."
 #endif
+
+/**
+ * Expand bit 0 of the given uint8_t to a mask_t all 1 or all 0
+ * The input must be either 0 or 1
+ */
+DECAF_INLINE mask_t bit_to_mask(uint8_t bit) {
+#ifdef _MSC_VER
+#pragma warning ( push)
+#pragma warning ( disable : 4146)
+#endif
+	return -bit;
+#ifdef _MSC_VER
+#pragma warning ( pop)
+#endif
+
+}
     
 /* Scalar limbs are keyed off of the API word size instead of the arch word size. */
 #if DECAF_WORD_BITS == 64
diff --git a/src/per_curve/decaf.tmpl.c b/src/per_curve/decaf.tmpl.c
index 89c1685..abdee61 100644
--- a/src/per_curve/decaf.tmpl.c
+++ b/src/per_curve/decaf.tmpl.c
@@ -1272,7 +1272,7 @@ decaf_error_t decaf_x$(gf_shortname) (
         if (t/8==0) sb &= -(uint8_t)COFACTOR;
         else if (t == X_PRIVATE_BITS-1) sb = -1;
         
-        mask_t k_t = ~((1 - ((sb>>(t%8)) & 1))*DECAF_MASK_ALL_SET); /* expand mask bit 0 to the whole mask without branching */
+        mask_t k_t = bit_to_mask((sb>>(t%8)) & 1);
         
         swap ^= k_t;
         gf_cond_swap(x2,x3,swap);
diff --git a/src/per_curve/elligator.tmpl.c b/src/per_curve/elligator.tmpl.c
index 98672de..0210d0a 100644
--- a/src/per_curve/elligator.tmpl.c
+++ b/src/per_curve/elligator.tmpl.c
@@ -109,13 +109,13 @@ API_NS(invert_elligator_nonuniform) (
     uint32_t hint_
 ) {
     mask_t hint = hint_;
-    mask_t sgn_s = ~((1 - (hint & 1))*DECAF_MASK_ALL_SET), /* expand hint bit 0 to the whole mask without branching */
-        sgn_altx = ~((1 - (hint>>1 & 1))*DECAF_MASK_ALL_SET),
-        sgn_r0 = ~((1 - (hint>>2 & 1))*DECAF_MASK_ALL_SET),
+    mask_t sgn_s = bit_to_mask(hint & 1),
+        sgn_altx = bit_to_mask((hint>>1) & 1),
+        sgn_r0 = bit_to_mask((hint>>2) & 1),
         /* FUTURE MAGIC: eventually if there's a curve which needs sgn_ed_T but not sgn_r0,
          * change this mask extraction.
          */
-        sgn_ed_T = ~((1 - (hint>>3 & 1))*DECAF_MASK_ALL_SET);
+        sgn_ed_T = bit_to_mask((hint>>3) & 1);
     gf a,b,c;
     API_NS(deisogenize)(a,b,c,p,sgn_s,sgn_altx,sgn_ed_T);
     
diff --git a/src/per_curve/scalar.tmpl.c b/src/per_curve/scalar.tmpl.c
index d95eb07..24d32cf 100644
--- a/src/per_curve/scalar.tmpl.c
+++ b/src/per_curve/scalar.tmpl.c
@@ -314,7 +314,7 @@ void API_NS(scalar_halve) (
     scalar_t out,
     const scalar_t a
 ) {
-    decaf_word_t mask = ~((1-(a->limb[0] & 1))*DECAF_WORD_ALL_SET); /* expand a->limb[0] bit 0 to the whole mask without branching */
+    decaf_word_t mask = bit_to_mask((a->limb[0]) & 1);
     decaf_dword_t chain = 0;
     unsigned int i;
     for (i=0; i<SCALAR_LIMBS; i++) {
diff --git a/src/per_field/f_generic.tmpl.c b/src/per_field/f_generic.tmpl.c
index 489f91a..33110f1 100644
--- a/src/per_field/f_generic.tmpl.c
+++ b/src/per_field/f_generic.tmpl.c
@@ -37,7 +37,7 @@ mask_t gf_lobit(const gf x) {
     gf y;
     gf_copy(y,x);
     gf_strong_reduce(y);
-    return ~((1-(y->limb[0]&1))*DECAF_MASK_ALL_SET); /* expand y->limb[0] bit 0 to the whole mask without branching */
+    return bit_to_mask((y->limb[0]) & 1);
 }
 
 /** Deserialize from wire format; return -1 on success and 0 on failure. */