https://bugs.llvm.org/show_bug.cgi?id=51975
Bug ID: 51975
Summary: Cost modelling for load-cast
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected],
[email protected]
We currently don't costmodel the fact that some casts
(mainly sext/zext, but also nt<->fp) can be folded into the load:
https://godbolt.org/z/vs7sx4nWz
This leads to rather pessimistic cost estimates,
and e.g. potentially affects LV.
Preceding question: does `X86TTIImpl::getCastInstrCost()`
have the right costs for e.g. `sext <8 x i16> to <8 x i32>`?
All `ISD::SIGN_EXTEND` in `AVX2ConversionTbl` are 2+,
for which target is that true?
I'd expect at least the s/zext's to cost 1.
Roughly, i suppose the solution matches that of the LOAD+BSWAP modelling,
i.e. if the cast is a single user of the load, then model it as free,
and when modelling the load which has a single cast user, then do ${MAGIC},
Where ${MAGIC} depends on the answer to the "Preceding question":
if the casts should cost 1, then we only need to adjust the load op sizes,
without increasing the load cost,
else i guess we need to report `max(load cost, cast cost)`?
Does this make sense?
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