[12.0.0]: x64: Remove recursion in to_amode helper
#6995
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This commit removes the recursion present in the x64 backend's `to_amode` and `to_amode_add` helpers. The recursion is currently unbounded and controlled by user input meaning it's not too hard to craft user input which triggers stack overflow in the host. By removing recursion there's no need to worry about this since the stack depth will never be hit. The main concern with removing recursion is that code quality may not be quite as good any more. The purpose of the recursion here is to "hunt for constants" and update the immediate `Offset32`, and now without recursion only at most one constant is found and folded instead of an arbitrary number of constants as before. This should continue to produce the same code as before so long as optimizations are enabled, but without optimizations this will produce worse code than before. Note with a hypothetical mid-end optimization that does this constant folding for us the rules here could be further simplified to purely consider the shape of the input `Value` to amode computation without considering constants at all.
alexcrichton
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pchickey
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Sep 11, 2023
alexcrichton
merged commit 9c2ed4e
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bytecodealliance:release-12.0.0
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veeshi
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Sep 19, 2023
This commit removes the recursion present in the x64 backend's `to_amode` and `to_amode_add` helpers. The recursion is currently unbounded and controlled by user input meaning it's not too hard to craft user input which triggers stack overflow in the host. By removing recursion there's no need to worry about this since the stack depth will never be hit. The main concern with removing recursion is that code quality may not be quite as good any more. The purpose of the recursion here is to "hunt for constants" and update the immediate `Offset32`, and now without recursion only at most one constant is found and folded instead of an arbitrary number of constants as before. This should continue to produce the same code as before so long as optimizations are enabled, but without optimizations this will produce worse code than before. Note with a hypothetical mid-end optimization that does this constant folding for us the rules here could be further simplified to purely consider the shape of the input `Value` to amode computation without considering constants at all.
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This is a backport of #6968 to the 12.0.0 release branch of Wasmtime. Note that AArch64 is not backported as its recursion is not present on this branch.