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update comment on branchless generation

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Brendan Hansknecht 2024-07-23 08:59:33 -07:00
parent 8316e3f9cd
commit 539ab1daef
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1 changed files with 3 additions and 4 deletions
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7
crates/compiler/builtins/bitcode/src/sort.zig
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@ -258,7 +258,7 @@ inline fn head_branchless_merge(dest: *[*]u8, left: *[*]u8, right: *[*]u8, cmp_d
// 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;
// TODO: double check this is branchless.
// 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.*);
@ -277,7 +277,7 @@ inline fn tail_branchless_merge(dest: *[*]u8, left: *[*]u8, right: *[*]u8, cmp_d
// *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;
// TODO: double check this is branchless.
// 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.*);
@ -290,8 +290,7 @@ inline fn tail_branchless_merge(dest: *[*]u8, left: *[*]u8, right: *[*]u8, cmp_d
/// 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;
// TODO: double check this is branchless. I would expect llvm to optimize this to be branchless.
// But based on reading some comments in quadsort, llvm seems to prefer branches very often.
// 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;