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405ff911da
- abs can only overflow, if a == MIN - comparing the sign change from wrapping addition is branchless - tests: MIN, MIN+1,..MIN+4, -42, -7, -1, 0, 1, 7.. See #1290
36 lines
1.0 KiB
Zig
36 lines
1.0 KiB
Zig
// absv - absolute oVerflow
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// * @panic, if value can not be represented
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// - absvXi4_generic for unoptimized version
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fn absvXi_generic(comptime ST: type) fn (a: ST) callconv(.C) ST {
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return struct {
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fn f(a: ST) callconv(.C) ST {
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const UT = switch (ST) {
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i32 => u32,
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i64 => u64,
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i128 => u128,
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else => unreachable,
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};
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// taken from Bit Twiddling Hacks
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// compute the integer absolute value (abs) without branching
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var x: ST = a;
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const N: UT = @bitSizeOf(ST);
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const sign: ST = a >> N - 1;
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x +%= sign;
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x ^= sign;
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if (x < 0)
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@panic("compiler_rt absv: overflow");
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return x;
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}
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}.f;
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}
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pub const __absvsi2 = absvXi_generic(i32);
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pub const __absvdi2 = absvXi_generic(i64);
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pub const __absvti2 = absvXi_generic(i128);
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test {
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_ = @import("absvsi2_test.zig");
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_ = @import("absvdi2_test.zig");
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_ = @import("absvti2_test.zig");
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}
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