Added detection for MIPS64 with n32 ABI.

[BearSSL] / src / int / i31_mulacc.c

diff --git a/src/int/i31_mulacc.c b/src/int/i31_mulacc.c
index 04a42c7..7410e54 100644 (file)
--- a/src/int/i31_mulacc.c
+++ b/src/int/i31_mulacc.c
@@ -29,14 +29,36 @@ void
 br_i31_mulacc(uint32_t *d, const uint32_t *a, const uint32_t *b)
 {
        size_t alen, blen, u;
+       uint32_t dl, dh;
 
        alen = (a[0] + 31) >> 5;
        blen = (b[0] + 31) >> 5;
-       d[0] = a[0] + b[0];
+
+       /*
+        * We want to add the two bit lengths, but these are encoded,
+        * which requires some extra care.
+        */
+       dl = (a[0] & 31) + (b[0] & 31);
+       dh = (a[0] >> 5) + (b[0] >> 5);
+       d[0] = (dh << 5) + dl + (~(uint32_t)(dl - 31) >> 31);
+
        for (u = 0; u < blen; u ++) {
                uint32_t f;
                size_t v;
+
+               /*
+                * Carry always fits on 31 bits; we want to keep it in a
+                * 32-bit register on 32-bit architectures (on a 64-bit
+                * architecture, cast down from 64 to 32 bits means
+                * clearing the high bits, which is not free; on a 32-bit
+                * architecture, the same operation really means ignoring
+                * the top register, which has negative or zero cost).
+                */
+#if BR_64
                uint64_t cc;
+#else
+               uint32_t cc;
+#endif
 
                f = b[1 + u];
                cc = 0;