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transform: assert that pointer-bearing selects do not reach escape analysis - #5882
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…alysis Escape analysis does not follow pointers through select instructions. It treats these uses as escaping, which can leave allocations on the heap. The passes before OptimizeAllocs do not currently produce these selects. Add an assertion to detect a pipeline change that breaks this assumption, rather than silently reducing allocation optimization.
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…ect (#5891) Escape analysis treats every select as an escape, so allocations that flow through a select stay on the heap even when nothing outside the function can see them. The frontend makes these selects for min and max on strings, and instcombine can make a select over a pointer before OptimizeAllocs runs. This came up in review of #5882, which tried to assert that no pointer-bearing select reaches escape analysis. That assumption is false, and the assertion crashes the compiler on valid Go code that calls min on a string that points to a local allocation. The better fix is to follow the select result in the same way as a bitcast, because the pointer can only leave through that result. The allocs2.go case does not show the stack allocation, because runtime.stringLess is only declared in that test. The IR cases in allocs-aggregate.ll show both the heap and the stack result.
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Escape analysis does not follow pointers through select instructions. It treats these uses as escaping, which can leave allocations on the heap.
The passes before OptimizeAllocs do not currently produce these selects. Add an assertion to detect a pipeline change that breaks this assumption, rather than silently reducing allocation optimization.