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opt_lut: always prefer to eliminate 1-LUTs. 

These are always either buffers or inverters, and keeping the larger
LUT preserves more source-level information about the design.
This commit is contained in:
whitequark 2018-12-05 13:14:44 +00:00
parent 59eea0183f
commit e603484070
1 changed files with 41 additions and 19 deletions
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60
passes/opt/opt_lut.cpp
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@ -154,35 +154,57 @@ struct OptLutWorker
int lutM_arity = lutA_arity + lutB_arity - 1 - common_inputs.size(); int lutM_arity = lutA_arity + lutB_arity - 1 - common_inputs.size();
log(" Cell A is a %d-LUT. Cell B is a %d-LUT. Cells share %zu input(s) and can be merged into one %d-LUT.\n", lutA_arity, lutB_arity, common_inputs.size(), lutM_arity); log(" Cell A is a %d-LUT. Cell B is a %d-LUT. Cells share %zu input(s) and can be merged into one %d-LUT.\n", lutA_arity, lutB_arity, common_inputs.size(), lutM_arity);
int combine = -1; const int COMBINE_A = 1, COMBINE_B = 2, COMBINE_EITHER = COMBINE_A | COMBINE_B;
if (combine == -1) int combine_mask = 0;
if (lutM_arity > lutA_width)
{ {
if (lutM_arity > lutA_width) log(" Not combining LUTs into cell A (combined LUT wider than cell A).\n");
{
log(" Not combining LUTs into cell A (combined LUT too wide).\n");
}
else if (lutB->get_bool_attribute("\\lut_keep"))
{
log(" Not combining LUTs into cell A (cell B has attribute \\lut_keep).\n");
}
else combine = 0;
} }
if (combine == -1) else if (lutB->get_bool_attribute("\\lut_keep"))
{ {
if (lutM_arity > lutB_width) log(" Not combining LUTs into cell A (cell B has attribute \\lut_keep).\n");
}
else
{
combine_mask |= COMBINE_A;
}
if (lutM_arity > lutB_width)
{
log(" Not combining LUTs into cell B (combined LUT wider than cell B).\n");
}
else if (lutA->get_bool_attribute("\\lut_keep"))
{
log(" Not combining LUTs into cell B (cell A has attribute \\lut_keep).\n");
}
else
{
combine_mask |= COMBINE_B;
}
int combine = combine_mask;
if (combine == COMBINE_EITHER)
{
log(" Can combine into either cell.\n");
if (lutA_arity == 1)
{ {
log(" Not combining LUTs into cell B (combined LUT too wide).\n"); log(" Cell A is a buffer or inverter, combining into cell B.\n");
combine = COMBINE_B;
} }
else if (lutA->get_bool_attribute("\\lut_keep")) else if (lutB_arity == 1)
{ {
log(" Not combining LUTs into cell B (cell A has attribute \\lut_keep).\n"); log(" Cell B is a buffer or inverter, combining into cell A.\n");
combine = COMBINE_A;
}
else
{
log(" Arbitrarily combining into cell A.\n");
combine = COMBINE_A;
} }
else combine = 1;
} }
RTLIL::Cell *lutM, *lutR; RTLIL::Cell *lutM, *lutR;
pool<SigBit> lutM_inputs, lutR_inputs; pool<SigBit> lutM_inputs, lutR_inputs;
if (combine == 0) if (combine == COMBINE_A)
{ {
log(" Combining LUTs into cell A.\n"); log(" Combining LUTs into cell A.\n");
lutM = lutA; lutM = lutA;
@ -190,7 +212,7 @@ struct OptLutWorker
lutR = lutB; lutR = lutB;
lutR_inputs = lutB_inputs; lutR_inputs = lutB_inputs;
} }
else if (combine == 1) else if (combine == COMBINE_B)
{ {
log(" Combining LUTs into cell B.\n"); log(" Combining LUTs into cell B.\n");
lutM = lutB; lutM = lutB;