finding the average of two unsigned integers without overflow<\/a>.<\/p>\nThe variable shifts have their own dedicated instructions.<\/p>\n
; logical shift left variable\r\n ; Wd = Wn << (Wm & 31)\r\n ; Xd = Xn << (Xm & 63)\r\n lslv Rd\/zr, Rn\/zr, Rm\/zr\r\n\r\n ; logical shift right variable\r\n ; Wd = Wn >> (Wm & 31), unsigned shift\r\n ; Xd = Xn >> (Xm & 63), unsigned shift\r\n lsrv Rd\/zr, Rn\/zr, Rm\/zr\r\n\r\n ; arithmetic shift right variable\r\n ; Wd = Wn >> (Wm & 31), signed shift\r\n ; Xd = Xn >> (Xm & 63), signed shift\r\n asrv Rd\/zr, Rn\/zr, Rm\/zr\r\n\r\n ; rotate right variable\r\n ; Rd = Rn rotated right by Rm positions\r\n rorv Rd\/zr, Rn\/zr, Rm\/zr\r\n<\/pre>\nNote that the shift amount is taken modulo the bit size of the operand. (This doesn’t really matter for RORV<\/code> since rotating by the operand bit size has no effect.)<\/p>\nThe pseudo-instructions LSL<\/code> LSR<\/code>, ASR<\/code>, and ROR<\/code> accept a register as the second input operand and convert it to the corresponding V<\/code> instruction. This means that when writing assembly, you can just write LSL<\/code> and let the assembler figure out which real opcode it corresponds to.<\/p>\nThere are no S<\/code> variants to the bit shifting instructions. They never update flags, unlike AArch32, which updated the carry with the last bit shifted out. If you want to know what bit got shifted out, you’ll have to calculate it yourself, say by shifting the same value again, but by one less position, and then inspecting the top\/bottom bit (depending on the shift direction).<\/p>\nI have my guesses as to why the designers removed the flags behavior from these instructions: First, it removes a partial register update (flags), which creates a usually-unwanted dependency on the previous flags. Second, no major programming language gives you access to the bit that was shifted out, so it wasn’t used in practice anyway.<\/p>\n
Exercise<\/b>: Suppose there was no double-register extraction instruction or variable rotation instruction. Synthesize fixed and variable rotation from other instructions. (Answer below.)<\/p>\n
Bonus chatter<\/b>: In AArch32, the bottom 8 bits of the shift-count register were used. But in AArch64, only the bottom 5 (for 32-bit operands) or 6 (for 64-bit operands) bits are used.<\/p>\n
Answer to exercise<\/b>: You can synthesize a fixed rotation from a shift and a bitfield insertion.<\/p>\n ; rotate r1 left by #imm, producing r0\r\n ; r1 = ABCDEFGH\r\n lsl r0, r1, #imm ; r0 = EFGH0000\r\n bfxil r0, r1, #(size-imm), #imm ; r0 = EFGHABCD\r\n<\/pre>\nA variable rotation can be synthesized from a pair of shifts.<\/p>\n
; rotate r1 left by r2, producing r0\r\n ; (destroys r2)\r\n ; r1 = ABCDEFGH\r\n lslv r0, r1, r2 ; r0 = EFGH0000\r\n mvn r2, r2 ; r2 = leftover bits\r\n lsrv r2, r1, r2 ; r2 = 0000ABCD\r\n orr r0, r0, r2 ; r0 = EFGHABCD\r\n<\/pre>\n\u00b9 Although it doesn’t explicitly have a “rotate left through carry” instruction, you can still do it in a single instruction:<\/p>\n
adcs r0, r1, r1 ; r0 = r1 rotated left through carry\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"Sliding around.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[2],"class_list":["post-106945","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-history"],"acf":[],"blog_post_summary":"
Sliding around.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/106945","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=106945"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/106945\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=106945"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=106945"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=106945"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}