diff options
Diffstat (limited to 'src/compiler/glsl/lower_instructions.cpp')
| -rw-r--r-- | src/compiler/glsl/lower_instructions.cpp | 106 |
1 files changed, 106 insertions, 0 deletions
diff --git a/src/compiler/glsl/lower_instructions.cpp b/src/compiler/glsl/lower_instructions.cpp index cfcc4100e98..4be60273e0d 100644 --- a/src/compiler/glsl/lower_instructions.cpp +++ b/src/compiler/glsl/lower_instructions.cpp @@ -164,6 +164,7 @@ private: void insert_to_shifts(ir_expression *); void reverse_to_shifts(ir_expression *ir); void find_lsb_to_float_cast(ir_expression *ir); + void find_msb_to_float_cast(ir_expression *ir); }; } /* anonymous namespace */ @@ -1311,6 +1312,106 @@ lower_instructions_visitor::find_lsb_to_float_cast(ir_expression *ir) this->progress = true; } +void +lower_instructions_visitor::find_msb_to_float_cast(ir_expression *ir) +{ + /* For more details, see: + * + * http://graphics.stanford.edu/~seander/bithacks.html#ZerosOnRightFloatCast + */ + const unsigned elements = ir->operands[0]->type->vector_elements; + ir_constant *c0 = new(ir) ir_constant(int(0), elements); + ir_constant *cminus1 = new(ir) ir_constant(int(-1), elements); + ir_constant *c23 = new(ir) ir_constant(int(23), elements); + ir_constant *c7F = new(ir) ir_constant(int(0x7F), elements); + ir_constant *c000000FF = new(ir) ir_constant(0x000000FFu, elements); + ir_constant *cFFFFFF00 = new(ir) ir_constant(0xFFFFFF00u, elements); + ir_variable *temp = + new(ir) ir_variable(glsl_type::uvec(elements), "temp", ir_var_temporary); + ir_variable *as_float = + new(ir) ir_variable(glsl_type::vec(elements), "as_float", ir_var_temporary); + ir_variable *msb = + new(ir) ir_variable(glsl_type::ivec(elements), "msb", ir_var_temporary); + + ir_instruction &i = *base_ir; + + i.insert_before(temp); + + if (ir->operands[0]->type->base_type == GLSL_TYPE_UINT) { + i.insert_before(assign(temp, ir->operands[0])); + } else { + assert(ir->operands[0]->type->base_type == GLSL_TYPE_INT); + + /* findMSB(uint(abs(some_int))) almost always does the right thing. + * There are two problem values: + * + * * 0x80000000. Since abs(0x80000000) == 0x80000000, findMSB returns + * 31. However, findMSB(int(0x80000000)) == 30. + * + * * 0xffffffff. Since abs(0xffffffff) == 1, findMSB returns + * 31. Section 8.8 (Integer Functions) of the GLSL 4.50 spec says: + * + * For a value of zero or negative one, -1 will be returned. + * + * For all negative number cases, including 0x80000000 and 0xffffffff, + * the correct value is obtained from findMSB if instead of negating the + * (already negative) value the logical-not is used. A conditonal + * logical-not can be achieved in two instructions. + */ + ir_variable *as_int = + new(ir) ir_variable(glsl_type::ivec(elements), "as_int", ir_var_temporary); + ir_constant *c31 = new(ir) ir_constant(int(31), elements); + + i.insert_before(as_int); + i.insert_before(assign(as_int, ir->operands[0])); + i.insert_before(assign(temp, i2u(expr(ir_binop_bit_xor, + as_int, + rshift(as_int, c31))))); + } + + /* The int-to-float conversion is lossless because bits are conditionally + * masked off the bottom of temp to ensure the value has at most 24 bits of + * data or is zero. We don't use the result in the zero case. The uint() + * cast is necessary so that 0x80000000 does not generate a negative value. + * + * float as_float = float(temp > 255 ? temp & ~255 : temp); + */ + i.insert_before(as_float); + i.insert_before(assign(as_float, u2f(csel(greater(temp, c000000FF), + bit_and(temp, cFFFFFF00), + temp)))); + + /* This is basically an open-coded frexp. Implementations that have a + * native frexp instruction would be better served by that. This is + * optimized versus a full-featured open-coded implementation in two ways: + * + * - We don't care about a correct result from subnormal numbers (including + * 0.0), so the raw exponent can always be safely unbiased. + * + * - The value cannot be negative, so it does not need to be masked off to + * extract the exponent. + * + * int msb = (floatBitsToInt(as_float) >> 23) - 0x7f; + */ + i.insert_before(msb); + i.insert_before(assign(msb, sub(rshift(bitcast_f2i(as_float), c23), c7F))); + + /* Use msb in the comparison instead of temp so that the subtract can + * possibly generate the result without an explicit comparison. + * + * (msb < 0) ? -1 : msb; + * + * Since our input values are all integers, the unbiased exponent must not + * be negative. It will only be negative (-0x7f, in fact) if temp is 0. + */ + ir->operation = ir_triop_csel; + ir->operands[0] = less(msb, c0); + ir->operands[1] = cminus1; + ir->operands[2] = new(ir) ir_dereference_variable(msb); + + this->progress = true; +} + ir_visitor_status lower_instructions_visitor::visit_leave(ir_expression *ir) { @@ -1438,6 +1539,11 @@ lower_instructions_visitor::visit_leave(ir_expression *ir) find_lsb_to_float_cast(ir); break; + case ir_unop_find_msb: + if (lowering(FIND_MSB_TO_FLOAT_CAST)) + find_msb_to_float_cast(ir); + break; + default: return visit_continue; } |