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Use CMOV on x86_64
CMOV has been constant time on all generations of x86_64 CPUs, even when reading from memory.
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@ -106,6 +106,36 @@ fe25519_neg(fe25519 h, const fe25519 f)
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Preconditions: b in {0,1}.
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*/
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#ifdef HAVE_AMD64_ASM
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static void
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fe25519_cmov(fe25519 f, const fe25519 g, unsigned int b)
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{
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uint64_t t64_0, t64_1, t64_2, t64_3, t64_4;
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__asm__ __volatile__(
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"cmpl $0, %[b] ;\n"
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"movq 0(%[f]), %[t64_0] ;\n"
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"movq 8(%[f]), %[t64_1] ;\n"
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"movq 16(%[f]), %[t64_2] ;\n"
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"movq 24(%[f]), %[t64_3] ;\n"
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"movq 32(%[f]), %[t64_4] ;\n"
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"cmovnzq 0(%[g]), %[t64_0] ;\n"
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"cmovnzq 8(%[g]), %[t64_1] ;\n"
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"cmovnzq 16(%[g]), %[t64_2] ;\n"
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"cmovnzq 24(%[g]), %[t64_3] ;\n"
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"cmovnzq 32(%[g]), %[t64_4] ;\n"
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"movq %[t64_0], 0(%[f]) ;\n"
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"movq %[t64_1], 8(%[f]) ;\n"
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"movq %[t64_2], 16(%[f]) ;\n"
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"movq %[t64_3], 24(%[f]) ;\n"
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"movq %[t64_4], 32(%[f]) ;\n"
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: [t64_0] "=&r"(t64_0), [t64_1] "=&r"(t64_1),
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[t64_2] "=&r"(t64_2), [t64_3] "=&r"(t64_3),
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[t64_4] "=&r"(t64_4)
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: [f] "r"(f), [g] "r"(g), [b] "rm"(b)
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: "memory", "flags", "cc");
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}
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#else
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static void
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fe25519_cmov(fe25519 f, const fe25519 g, unsigned int b)
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{
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@ -135,6 +165,7 @@ fe25519_cmov(fe25519 f, const fe25519 g, unsigned int b)
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f[3] = f3 ^ x3;
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f[4] = f4 ^ x4;
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}
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#endif
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/*
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Replace (f,g) with (g,f) if b == 1;
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