Cranelift: fix regalloc2 integration bug wrt blockparam branch args. #4042
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Previously, the block successor accumulation and the blockparam branch arg setup were decoupled. The lowering backend implicitly specified the order of successor edges via its `MachTerminator` enum on the last instruction in the block, while the `Lower` toplevel machine-independent driver set up blockparam branch args in the edge order seen in CLIF. In some cases, these orders did not match -- for example, when the conditional branch depended on an FP condition that was implemented by swapping taken/not-taken edges and inverting the condition code. This PR refactors the successor handling to be centralized in `Lower` rather than flow through the terminator `MachInst`, and adds a successor block and its blockparam args at the same time, ensuring the orders match.
cfallin
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This requires bytecodealliance/regalloc2#43, and is stacked on top of PR bytecodealliance#4042, a bug that this integration helped to find. With these fixes, all this PR has to do is instantiate and run the checker on the `regalloc2::Output`. This is off by default, and is enabled by setting the `regalloc_checker` Cranelift option. This restores the old functionality provided by e.g. the `backtracking_checked` regalloc algorithm setting rather than `backtracking` when we were still on regalloc.rs.
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Previously, the block successor accumulation and the blockparam branch
arg setup were decoupled. The lowering backend implicitly specified
the order of successor edges via its
MachTerminatorenum on the lastinstruction in the block, while the
Lowertoplevelmachine-independent driver set up blockparam branch args in the edge
order seen in CLIF.
In some cases, these orders did not match -- for example, when the
conditional branch depended on an FP condition that was implemented by
swapping taken/not-taken edges and inverting the condition code.
This PR refactors the successor handling to be centralized in
Lowerrather than flow through the terminator
MachInst, and adds asuccessor block and its blockparam args at the same time, ensuring the
orders match.