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stage1: fix comptime saturation subtraction

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- also simplifies code
- adding a few more tests

closes #10870
This commit is contained in:
Jan Philipp Hafer 2022-02-15 01:43:59 +01:00 • committed by Veikka Tuominen
parent be98f30a2d
commit c6cd919a18
2 changed files with 34 additions and 15 deletions
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28
src/stage1/bigint.cpp
View file

@ -476,9 +476,15 @@ void bigint_min(BigInt* dest, const BigInt *op1, const BigInt *op2) {
/// signed bounds are [-2^(bit_count-1)..2^(bit_count-1)-1]
/// unsigned bounds are [0..2^bit_count-1]
void bigint_clamp_by_bitcount(BigInt* dest, uint32_t bit_count, bool is_signed) {
bool is_negative = dest->is_negative;
// unsigned and dest->is_negative => clamp to 0
if (is_negative && !is_signed) {
bigint_deinit(dest);
bigint_init_unsigned(dest, 0);
return;
}
// compute the number of bits required to store the value, and use that
// to decide whether to clamp the result
bool is_negative = dest->is_negative;
// to workaround the fact this bits_needed calculation would yield 65 or more for
// all negative numbers, set is_negative to false. this is a cheap way to find
// bits_needed(abs(dest)).
@ -512,19 +518,13 @@ void bigint_clamp_by_bitcount(BigInt* dest, uint32_t bit_count, bool is_signed)
*dest = bound_sub_one;
}
} else {
if(is_negative) {
bigint_deinit(dest);
bigint_init_unsigned(dest, 0);
return; // skips setting is_negative which would be invalid
} else {
BigInt bound;
bigint_shl(&bound, &one, &bit_count_big);
BigInt bound_sub_one;
bigint_sub(&bound_sub_one, &bound, &one);
bigint_deinit(&bound);
bigint_deinit(dest);
*dest = bound_sub_one;
}
BigInt bound;
bigint_shl(&bound, &one, &bit_count_big);
BigInt bound_sub_one;
bigint_sub(&bound_sub_one, &bound, &one);
bigint_deinit(&bound);
bigint_deinit(dest);
*dest = bound_sub_one;
}
}
dest->is_negative = is_negative;

21
test/behavior/saturating_arithmetic.zig
View file

@ -8,12 +8,17 @@ test "saturating add" {
const S = struct {
fn doTheTest() !void {
try testSatAdd(i8, -3, 10, 7);
try testSatAdd(i8, 3, -10, -7);
try testSatAdd(i8, -128, -128, -128);
try testSatAdd(i2, 1, 1, 1);
try testSatAdd(i2, 1, -1, 0);
try testSatAdd(i2, -1, -1, -2);
try testSatAdd(i64, maxInt(i64), 1, maxInt(i64));
try testSatAdd(i128, maxInt(i128), -maxInt(i128), 0);
try testSatAdd(i128, minInt(i128), maxInt(i128), -1);
try testSatAdd(i8, 127, 127, 127);
try testSatAdd(u2, 0, 0, 0);
try testSatAdd(u2, 0, 1, 1);
try testSatAdd(u8, 3, 10, 13);
try testSatAdd(u8, 255, 255, 255);
try testSatAdd(u2, 3, 2, 3);
@ -34,7 +39,11 @@ test "saturating add" {
comptime try S.doTheTest();
comptime try S.testSatAdd(comptime_int, 0, 0, 0);
comptime try S.testSatAdd(comptime_int, -1, 1, 0);
comptime try S.testSatAdd(comptime_int, 3, 2, 5);
comptime try S.testSatAdd(comptime_int, -3, -2, -5);
comptime try S.testSatAdd(comptime_int, 3, -2, 1);
comptime try S.testSatAdd(comptime_int, -3, 2, -1);
comptime try S.testSatAdd(comptime_int, 651075816498665588400716961808225370057, 468229432685078038144554201546849378455, 1119305249183743626545271163355074748512);
comptime try S.testSatAdd(comptime_int, 7, -593423721213448152027139550640105366508, -593423721213448152027139550640105366501);
}
@ -43,14 +52,20 @@ test "saturating subtraction" {
const S = struct {
fn doTheTest() !void {
try testSatSub(i8, -3, 10, -13);
try testSatSub(i8, -3, -10, 7);
try testSatSub(i8, -128, -128, 0);
try testSatSub(i8, -1, 127, -128);
try testSatSub(i2, 1, 1, 0);
try testSatSub(i2, 1, -1, 1);
try testSatSub(i2, -2, -2, 0);
try testSatSub(i64, minInt(i64), 1, minInt(i64));
try testSatSub(i128, maxInt(i128), -1, maxInt(i128));
try testSatSub(i128, minInt(i128), -maxInt(i128), -1);
try testSatSub(u2, 0, 0, 0);
try testSatSub(u2, 0, 1, 0);
try testSatSub(u5, 0, 31, 0);
try testSatSub(u8, 10, 3, 7);
try testSatSub(u8, 0, 255, 0);
try testSatSub(u5, 0, 31, 0);
try testSatSub(u128, 0, maxInt(u128), 0);
}
@ -67,7 +82,11 @@ test "saturating subtraction" {
comptime try S.doTheTest();
comptime try S.testSatSub(comptime_int, 0, 0, 0);
comptime try S.testSatSub(comptime_int, 1, 1, 0);
comptime try S.testSatSub(comptime_int, 3, 2, 1);
comptime try S.testSatSub(comptime_int, -3, -2, -1);
comptime try S.testSatSub(comptime_int, 3, -2, 5);
comptime try S.testSatSub(comptime_int, -3, 2, -5);
comptime try S.testSatSub(comptime_int, 651075816498665588400716961808225370057, 468229432685078038144554201546849378455, 182846383813587550256162760261375991602);
comptime try S.testSatSub(comptime_int, 7, -593423721213448152027139550640105366508, 593423721213448152027139550640105366515);
}