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improve branchless code gen

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Brendan Hansknecht 2024-07-23 10:46:27 -07:00
parent 1d534517b3
commit 9702cae99d
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1 changed files with 14 additions and 24 deletions
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38
crates/compiler/builtins/bitcode/src/sort.zig
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@ -261,18 +261,13 @@ inline fn parity_merge_two(ptr: [*]u8, swap: [*]u8, cmp_data: Opaque, cmp: Compa
/// Inlining will remove the extra level of pointer indirection here.
/// It is just used to allow mutating the input pointers.
inline fn head_branchless_merge(dest: *[*]u8, left: *[*]u8, right: *[*]u8, cmp_data: Opaque, cmp: CompareFn, element_width: usize, copy: CopyFn) void {
// Note there is a much simpler version here:
// Note equivalent c code:
// *ptd++ = cmp(ptl, ptr) <= 0 ? *ptl++ : *ptr++;
// That said, not sure how to write that in zig and guarantee it is branchless.
// Thus using the longer form.
const lte = compare(cmp, cmp_data, left.*, right.*) != GT;
// While not guaranteed branchless, tested in godbolt for x86_64, aarch32, aarch64, riscv64, and wasm32.
const x = if (lte) element_width else 0;
const not_x = if (lte) 0 else element_width;
copy(dest.*, left.*);
left.* += x;
copy((dest.* + x), right.*);
right.* += not_x;
const lte = compare(cmp, cmp_data, left.*, right.*) != GT;
const from = if (lte) left else right;
copy(dest.*, from.*);
from.* += element_width;
dest.* += element_width;
}
@ -281,28 +276,23 @@ inline fn head_branchless_merge(dest: *[*]u8, left: *[*]u8, right: *[*]u8, cmp_d
/// Inlining will remove the extra level of pointer indirection here.
/// It is just used to allow mutating the input pointers.
inline fn tail_branchless_merge(dest: *[*]u8, left: *[*]u8, right: *[*]u8, cmp_data: Opaque, cmp: CompareFn, element_width: usize, copy: CopyFn) void {
// Note there is a much simpler version here:
// Note equivalent c code:
// *tpd-- = cmp(tpl, tpr) > 0 ? *tpl-- : *tpr--;
// That said, not sure how to write that in zig and guarantee it is branchless.
const lte = compare(cmp, cmp_data, left.*, right.*) != GT;
// While not guaranteed branchless, tested in godbolt for x86_64, aarch32, aarch64, riscv64, and wasm32.
const y = if (lte) element_width else 0;
const not_y = if (lte) 0 else element_width;
copy(dest.*, left.*);
left.* -= not_y;
const gt = compare(cmp, cmp_data, left.*, right.*) == GT;
const from = if (gt) left else right;
copy(dest.*, from.*);
from.* -= element_width;
dest.* -= element_width;
copy((dest.* + y), right.*);
right.* -= y;
}
/// Swaps the element at ptr with the element after it if the element is greater than the next.
inline fn swap_branchless(ptr: [*]u8, swap: [*]u8, cmp_data: Opaque, cmp: CompareFn, element_width: usize, copy: CopyFn) void {
const gt = compare(cmp, cmp_data, ptr, ptr + element_width) == GT;
// While not guaranteed branchless, tested in godbolt for x86_64, aarch32, aarch64, riscv64, and wasm32.
const x = if (gt) element_width else 0;
const y = if (gt) 0 else element_width;
copy(swap, ptr + y);
const gt = compare(cmp, cmp_data, ptr, ptr + element_width) == GT;
var x = if (gt) element_width else 0;
const from = if (gt) ptr else ptr + element_width;
copy(swap, from);
copy(ptr, ptr + x);
copy(ptr + element_width, swap);
}