[go] cmd/compile: eliminate redundant sign-extensions for wasm

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Gopher Robot (Gerrit)

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Mar 17, 2026, 2:49:32 PM (yesterday) Mar 17

to George Adams, goph...@pubsubhelper.golang.org, golang-...@googlegroups.com, Carlos Amedee, Keith Randall, Keith Randall, Go LUCI, Quim Muntal, David Chase, golang-co...@googlegroups.com

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Change information

Commit message:

cmd/compile: eliminate redundant sign-extensions for wasm

Add rules to eliminate redundant I64Extend sign-extension operations
in the wasm backend:

Idempotent (applying the same extend twice is redundant):

  (I64Extend32S (I64Extend32S x)) => (I64Extend32S x)
  (I64Extend16S (I64Extend16S x)) => (I64Extend16S x)
  (I64Extend8S  (I64Extend8S  x)) => (I64Extend8S  x)

Narrower-subsumes-wider (a narrower sign-extend already determines
all the bits that a wider one would set):

  (I64Extend32S (I64Extend16S x)) => (I64Extend16S x)
  (I64Extend32S (I64Extend8S  x)) => (I64Extend8S  x)
  (I64Extend16S (I64Extend8S  x)) => (I64Extend8S  x)

These patterns arise from nested sub-word type conversions. For
example, converting int8 -> int16 -> int32 -> int64 lowers to
I64Extend8S -> I64Extend16S -> I64Extend32S, but the I64Extend8S
alone is sufficient since it already sign-extends from 8 to 64 bits.