Implement x64 16-bit bswap with rol (bytecodealliance#1092)

x64 bswap instruction is only for 32- or 64-bit registers, so implement the 16-bit swap as a rotate left by 8
11evan · Oct 26, 2022 · e1f1dde · e1f1dde
1 parent ec502b5
commit e1f1dde
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Showing 2 changed files with 7 additions and 4 deletions.
diff --git a/cranelift/codegen/src/isa/x64/inst.isle b/cranelift/codegen/src/isa/x64/inst.isle
@@ -151,7 +151,6 @@
        (Setcc (cc CC)
               (dst WritableGpr))
 
-       ;; TODO xchg impl needed for 2 byte swap
        ;; Swaps byte order in register
        (Bswap (size OperandSize) ;; 4 or 8
               (src Gpr)
@@ -1972,9 +1971,8 @@
       (shift_r ty (ShiftKind.ShiftRightArithmetic) src1 src2))
 
 ;; Helper for creating byteswap instructions.
-;; In x64, 32- and 64-bit registers use BSWAP instruction, however
-;; this instruction is undefined for 16-bit registers, which instead
-;; use XCHG
+;; In x64, 32- and 64-bit registers use BSWAP instruction, and
+;; for 16-bit registers one must instead use xchg or rol/ror
 (decl x64_bswap (Type Gpr) Gpr)
 (rule (x64_bswap ty src)
       (let ((dst WritableGpr (temp_writable_gpr))

diff --git a/cranelift/codegen/src/isa/x64/lower.isle b/cranelift/codegen/src/isa/x64/lower.isle
@@ -2046,6 +2046,11 @@
 
 ;; Rules for `bswap` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
+;; x64 bswap instruction is only for 32- or 64-bit swaps
+;; implement the 16-bit swap as a rotl by 8
+(rule (lower (has_type $I16 (bswap src)))
+      (x64_rotl $I16 src (Imm8Reg.Imm8 8)))
+
 (rule (lower (has_type $I32 (bswap src)))
       (x64_bswap $I32 src))