45 lines
1.1 KiB
Zig
45 lines
1.1 KiB
Zig
const builtin = @import("builtin");
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const compiler_rt = @import("../compiler_rt.zig");
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const maxInt = std.math.maxInt;
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const minInt = std.math.minInt;
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pub extern fn __mulodi4(a: i64, b: i64, overflow: *c_int) i64 {
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@setRuntimeSafety(builtin.is_test);
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const min = @bitCast(i64, u64(1 << (i64.bit_count - 1)));
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const max = ~min;
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overflow.* = 0;
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const result = a *% b;
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// Edge cases
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if (a == min) {
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if (b != 0 and b != 1) overflow.* = 1;
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return result;
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}
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if (b == min) {
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if (a != 0 and a != 1) overflow.* = 1;
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return result;
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}
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// Take absolute value of a and b via abs(x) = (x^(x >> 63)) - (x >> 63).
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const abs_a = (a ^ (a >> 63)) -% (a >> 63);
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const abs_b = (b ^ (b >> 63)) -% (b >> 63);
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// Unitary magnitude, cannot have overflow
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if (abs_a < 2 or abs_b < 2) return result;
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// Compare the signs of the operands
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if ((a ^ b) >> 63 != 0) {
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if (abs_a > @divTrunc(max, abs_b)) overflow.* = 1;
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} else {
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if (abs_a > @divTrunc(min, -abs_b)) overflow.* = 1;
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}
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return result;
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}
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test "import mulodi4" {
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_ = @import("mulodi4_test.zig");
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}
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