From: Eduardo Lima Mitev Date: Thu, 28 Feb 2019 17:17:50 +0000 (+0100) Subject: ir3/lower_io_offsets: Try propagate SSBO's SHR into a previous shift instruction X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=759ceda07e122e3b2cc7a5e44698f41accb5e92c;p=mesa.git ir3/lower_io_offsets: Try propagate SSBO's SHR into a previous shift instruction While we lack value range tracking, this patch tries to 'manually' propogate the division by 4 to calculate SSBO element-offset, into a possible previous shift operation (shift left or right); checking that it is safe to do so. This should help in cases like ie. when accessing a field in an array of structs, where the offset is likely defined as base plus a multiplication by a struct or array element size. See dEQP test 'dEQP-GLES31.functional.ssbo.atomic.xor.highp_uint' for an example of a shader that benefits from this. Reviewed-by: Rob Clark --- diff --git a/src/freedreno/ir3/ir3_nir_lower_io_offsets.c b/src/freedreno/ir3/ir3_nir_lower_io_offsets.c index 264f1064545..bc50fa09f3b 100644 --- a/src/freedreno/ir3/ir3_nir_lower_io_offsets.c +++ b/src/freedreno/ir3/ir3_nir_lower_io_offsets.c @@ -83,6 +83,81 @@ get_ir3_intrinsic_for_ssbo_intrinsic(unsigned intrinsic, return -1; } +static nir_ssa_def * +check_and_propagate_bit_shift32(nir_builder *b, nir_ssa_def *offset, + nir_alu_instr *alu_instr, int32_t direction, + int32_t shift) +{ + debug_assert(alu_instr->src[1].src.is_ssa); + nir_ssa_def *shift_ssa = alu_instr->src[1].src.ssa; + + /* Only propagate if the shift is a const value so we can check value range + * statically. + */ + nir_const_value *const_val = nir_src_as_const_value(alu_instr->src[1].src); + if (!const_val) + return NULL; + + int32_t current_shift = const_val->i32[0] * direction; + int32_t new_shift = current_shift + shift; + + /* If the merge would reverse the direction, bail out. + * e.g, 'x << 2' then 'x >> 4' is not 'x >> 2'. + */ + if (current_shift * new_shift < 0) + return NULL; + + /* If the propagation would overflow an int32_t, bail out too to be on the + * safe side. + */ + if (new_shift < -31 || new_shift > 31) + return NULL; + + b->cursor = nir_before_instr(&alu_instr->instr); + + /* Add or substract shift depending on the final direction (SHR vs. SHL). */ + if (shift * direction < 0) + shift_ssa = nir_isub(b, shift_ssa, nir_imm_int(b, abs(shift))); + else + shift_ssa = nir_iadd(b, shift_ssa, nir_imm_int(b, abs(shift))); + + return shift_ssa; +} + +static nir_ssa_def * +try_propagate_bit_shift(nir_builder *b, nir_ssa_def *offset, int32_t shift) +{ + nir_instr *offset_instr = offset->parent_instr; + if (offset_instr->type != nir_instr_type_alu) + return NULL; + + nir_alu_instr *alu = nir_instr_as_alu(offset_instr); + nir_ssa_def *shift_ssa; + nir_ssa_def *new_offset = NULL; + + switch (alu->op) { + case nir_op_ishl: + shift_ssa = check_and_propagate_bit_shift32(b, offset, alu, 1, shift); + if (shift_ssa) + new_offset = nir_ishl(b, alu->src[0].src.ssa, shift_ssa); + break; + case nir_op_ishr: + shift_ssa = check_and_propagate_bit_shift32(b, offset, alu, -1, shift); + if (shift_ssa) + new_offset = nir_ishr(b, alu->src[0].src.ssa, shift_ssa); + break; + case nir_op_ushr: + shift_ssa = check_and_propagate_bit_shift32(b, offset, alu, -1, shift); + if (shift_ssa) + new_offset = nir_ushr(b, alu->src[0].src.ssa, shift_ssa); + break; + default: + return NULL; + } + + return new_offset; +} + static bool lower_offset_for_ssbo(nir_intrinsic_instr *intrinsic, nir_builder *b, unsigned ir3_ssbo_opcode, uint8_t offset_src_idx) @@ -104,6 +179,16 @@ lower_offset_for_ssbo(nir_intrinsic_instr *intrinsic, nir_builder *b, debug_assert(intrinsic->src[offset_src_idx].is_ssa); nir_ssa_def *offset = intrinsic->src[offset_src_idx].ssa; + /* Since we don't have value range checking, we first try to propagate + * the division by 4 ('offset >> 2') into another bit-shift instruction that + * possibly defines the offset. If that's the case, we emit a similar + * instructions adjusting (merging) the shift value. + * + * Here we use the convention that shifting right is negative while shifting + * left is positive. So 'x / 4' ~ 'x >> 2' or 'x << -2'. + */ + nir_ssa_def *new_offset = try_propagate_bit_shift(b, offset, -2); + /* The new source that will hold the dword-offset is always the last * one for every intrinsic. */ @@ -127,11 +212,16 @@ lower_offset_for_ssbo(nir_intrinsic_instr *intrinsic, nir_builder *b, new_intrinsic->num_components = intrinsic->num_components; b->cursor = nir_before_instr(&intrinsic->instr); - nir_ssa_def *offset_div_4 = nir_ushr(b, offset, nir_imm_int(b, 2)); - debug_assert(offset_div_4); + + /* If we managed to propagate the division by 4, just use the new offset + * register and don't emit the SHR. + */ + if (new_offset) + offset = new_offset; + else + offset = nir_ushr(b, offset, nir_imm_int(b, 2)); /* Insert the new intrinsic right before the old one. */ - b->cursor = nir_before_instr(&intrinsic->instr); nir_builder_instr_insert(b, &new_intrinsic->instr); /* Replace the last source of the new intrinsic by the result of @@ -139,7 +229,7 @@ lower_offset_for_ssbo(nir_intrinsic_instr *intrinsic, nir_builder *b, */ nir_instr_rewrite_src(&new_intrinsic->instr, target_src, - nir_src_for_ssa(offset_div_4)); + nir_src_for_ssa(offset)); if (has_dest) { /* Replace the uses of the original destination by that