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hk: clarify bounds check calculations

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Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32434>
This commit is contained in:
Alyssa Rosenzweig 2024-11-27 17:17:21 -05:00 • committed by Marge Bot
parent 4d4f82b739
commit 2b3b0cd339
1 changed files with 24 additions and 14 deletions
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38
src/asahi/vulkan/hk_shader.c
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@ -300,22 +300,32 @@ lower_load_global_constant_offset_instr(nir_builder *b,
if (*has_soft_fault) {
nir_scalar offs = nir_scalar_resolved(offset, 0);
if (nir_scalar_is_const(offs)) {
unsigned offs_imm = nir_scalar_as_uint(offs);
/* Simplify the bounds check */
/* Calculate last byte loaded */
unsigned offs_imm = nir_scalar_as_uint(offs) + load_size;
/* Simplify the bounds check. Uniform buffers are bounds checked at
* 64B granularity, so `bound` is a multiple of K = 64. Then
*
* offs_imm < bound <==> round_down(offs_imm, K) < bound. Proof:
*
* "=>" round_down(offs_imm, K) <= offs_imm < bound.
*
* "<=" Let a, b be integer s.t. offs_imm = K a + b with b < K.
* Note round_down(offs_imm, K) = Ka.
*
* Let c be integer s.t. bound = Kc.
* We have Ka < Kc => a < c.
* b < K => Ka + b < K(a + 1).
*
* a < c with integers => a + 1 <= c.
* offs_imm < K(a + 1) <= Kc = bound.
* Hence offs_imm < bound.
*/
assert(align_mul == 64);
offs_imm &= ~(align_mul - 1);
/* In hk_buffer_addr_range, we ensure that zero-sized buffers get
* address 0. Why? Suppose offs_imm == 0.
*
* If the buffer is zero-sized, this is out-of-bounds. The above
* driver ABI ensures the calculated address is 0 + 0 == 0,
* returning zero
*
* Otherwise, the buffer is not zero-sized. For sufficiently large
* robustness granularity, that means the address is necessarily
* in-bounds.
*
* In both cases, the bounds check is unnecessary.
/* Bounds checks are `offset > bound ? 0 : val` so if offset = 0,
* the bounds check is useless.
*/
if (offs_imm) {
val = bounds_check(b, val, nir_imm_int(b, offs_imm), bound);