cmd/internal/obj/arm64: add a restriction on move constant instructions

For MOVK/MOVN/MOVZ instructions, the assembler does not accept zero
shifts, and the CL 275812 added the error check. This CL adds this
restriction to the document.

Change-Id: I8818d76ca2f11dade2307f3678ca521f4e64d164
Reviewed-on: https://go-review.googlesource.com/c/go/+/312210
Trust: fannie zhang <Fannie.Zhang@arm.com>
Run-TryBot: fannie zhang <Fannie.Zhang@arm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

diff --git a/src/cmd/internal/obj/arm64/doc.go b/src/cmd/internal/obj/arm64/doc.go
index efd4577f56bae0c8c65813df5756890a7c4c1cea..14f0f4c616f227e9b4175195152d8d817b726e5e 100644 (file)
--- a/src/cmd/internal/obj/arm64/doc.go
+++ b/src/cmd/internal/obj/arm64/doc.go
@@ -96,6 +96,19 @@ And for a 128-bit interger, it take two 64-bit operands, for the high and low pa
     VMOVD $0x1122334455667788, V1
     VMOVQ $0x1122334455667788, $8877665544332211, V2   // V2=0x11223344556677888877665544332211
 
+8. Move an optionally-shifted 16-bit immediate value to a register.
+
+The instructions are MOVK(W), MOVZ(W) and MOVN(W), the assembly syntax is "op $(uimm16<<shift), <Rd>". The <uimm16>
+is the 16-bit unsigned immediate, in the range 0 to 65535; For the 32-bit variant, the <shift> is 0 or 16, for the
+64-bit variant, the <shift> is 0, 16, 32 or 48.
+
+The current Go assembler does not accept zero shifts, such as "op $0, Rd" and "op $(0<<(16|32|48)), Rd" instructions.
+
+  Examples:
+    MOVK $(10<<32), R20     <=>      movk x20, #10, lsl #32
+    MOVZW $(20<<16), R8     <=>      movz w8, #20, lsl #16
+    MOVK $(0<<16), R10 will be reported as an error by the assembler.
+
 Special Cases.
 
 (1) umov is written as VMOV.