sdesmalen-arm wrote:
The assumption cache mechanism is used by a number of passes, such as [partial]
inlining, function specialization and IPSCCP (interprocedural sparse
conditional constant propagation).
The idea behind doing this is to let optimizations iteratively apply knowledge
about the streaming-mode of the caller when analyzing or optimizing a callee.
If we'd move this functionality to e.g. constant-folding, then we can only
apply this knowledge on functions where the streaming-mode is already known. If
we'd combine the intrinsic in the instcombine pass, then this is combined only
once, instead of iteratively by a pass that needs the information while e.g.
inlining.
The idea of using llvm.assume is to give the compiler more knowledge when
trying to inline or specialize a function. For example, if the callee has a `if
(__arm_in_streaming_mode()) { ... }` branch, then the cost-model could infer
that this is always/never executed, depending on mode of the call-site, which
changes the cost/decision on whether or not to inline. IIRC, when it inlines
the IR cloner also simplifies the code based on the assumption cache.
I understand that emitting this on a per-call basis is a bit of a hammer. I had
expected/hoped that LLVM passes would hoist/CSE the multiple llvm.assume's to
reduce the number of these intrinsics (because the value of the condition is
constant inside the function). Am I correct in thinking that emitting an
llvm.assume once in the entry block of a function is not a problem?
https://github.com/llvm/llvm-project/pull/121917
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