[FirRegLowering] Add limit to number of ifs generated #8313
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When lowering a firreg to SV, we structure the IR so that the register is driven from inside an if-op tree. We build this structure by examining the register's next value, translating mux ops into if-ops. This PR sets a limit of 1024 on the number of if-ops generated for each register. While it is unlikely to hit this limit in the common case, it is possible to cause firtool to produce a very large if-op tree from a relatively small number of mux ops. The goal is to prevent firtool from hanging. If we exceed the limit, we stop translating muxes to nested if-ops, and just drive the register with the original mux op result instead. This PR changes the work stack to a work queue, so that we create if-ops in a breadth-first order, so that the budget is allocated evenly across the branches of the if-op tree.
KelvinChung2000
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Apr 22, 2025
When lowering a firreg to SV, we structure the IR so that the register is driven from inside an if-op tree. We build this structure by examining the register's next value, translating mux ops into if-ops. This PR sets a limit of 1024 on the number of if-ops generated for each register. While it is unlikely to hit this limit in the common case, it is possible to cause firtool to produce a very large if-op tree from a relatively small number of mux ops. The goal is to prevent firtool from hanging. If we exceed the limit, we stop translating muxes to nested if-ops, and just drive the register with the original mux op result instead. This PR changes the work stack to a work queue, so that we create if-ops in a breadth-first order, so that the budget is allocated evenly across the branches of the if-op tree.
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When lowering a firreg to SV, we structure the IR so that the register is driven from inside an if-op tree. We build this structure by examining the register's next value, translating mux ops into if-ops.
This PR sets a limit of 1024 on the number of if-ops generated for each register. While it is unlikely to hit this limit in the common case, it is possible to cause firtool to produce a very large if-op tree from a relatively small number of mux ops.
If we exceed the limit, we stop translating muxes to nested if-ops, and just drive the register with the original mux op result instead.
This PR changes the work stack to a work queue, so that we create if-ops in a breadth-first order, so that the budget is allocated evenly across the branches of the if-op tree.