T0b1/wasmtime
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

Cranelift: Rewrite (x>>k)<<k into masking off the bottom k bits (#5673)

Browse Source 
* Cranelift: Rewrite `(x>>k)<<k` into masking off the bottom `k` bits

* Add a runtest for exercising our rewrite of `(x >> k) << k` into masking
This commit is contained in:
Nick Fitzgerald
2023-01-31 13:11:12 -08:00
committed by GitHub
parent 7f2c8e6344
commit 253e28ca4f
3 changed files with 153 additions and 0 deletions
Download Patch File Download Diff File Expand all files Collapse all files

13
cranelift/codegen/src/opts/algebraic.isle
View File

@@ -160,6 +160,19 @@
;; TODO: strength reduction: div to shifts
;; TODO: div/rem by constants -> magic multiplications
;; `(x >> k) << k` is the same as masking off the bottom `k` bits (regardless if
;; this is a signed or unsigned shift right).
(rule (simplify (ishl (fits_in_64 ty)
(ushr ty x (iconst _ (u64_from_imm64 k)))
(iconst _ (u64_from_imm64 k))))
(let ((mask u64 (u64_shl 0xFFFFFFFFFFFFFFFF k)))
(band ty x (iconst ty (imm64_masked ty mask)))))
(rule (simplify (ishl (fits_in_64 ty)
(sshr ty x (iconst _ (u64_from_imm64 k)))
(iconst _ (u64_from_imm64 k))))
(let ((mask u64 (u64_shl 0xFFFFFFFFFFFFFFFF k)))
(band ty x (iconst ty (imm64_masked ty mask)))))
;; Rematerialize ALU-op-with-imm and iconsts in each block where they're
;; used. This is neutral (add-with-imm) or positive (iconst) for
;; register pressure, and these ops are very cheap.