Hi,
In http://felleisen.org/matthias/manifesto/, you can read:
"In support, Racket offers protection mechanisms to implement a full
language spectrum, from C-level bit manipulation to soundly typed
extensions."
What are we talking about here when we mention C-level bit manipulation?
Is this referring to the ffi or some other Racket feature that allows me
to generate unsafe bit manipulation instructions?
In C, for a bitwise_and of an integer x and 0xff, I get:
https://godbolt.org/g/gy3ZN9
bitwise_and:
movzx eax, dil
ret
In Racket, I tried:
#lang racket
(require disassemble
racket/unsafe/ops)
(define (lsb x)
(bitwise-and x #xff))
(disassemble lsb)
0: 488b4808 (mov rcx (mem64+ rax #x8))
4: 48898bf8ffffff (mov (mem64+ rbx #x-8) rcx)
b: 488b4810 (mov rcx (mem64+ rax #x10))
f: 48898bf0ffffff (mov (mem64+ rbx #x-10) rcx)
16: 4881c3f0ffffff (add rbx #xfffffffffffffff0)
1d: 488b13 (mov rdx (mem64+ rbx))
20: 488b5230 (mov rdx (mem64+ rdx #x30))
24: 488b4208 (mov rax (mem64+ rdx #x8))
28: 488983f8ffffff (mov (mem64+ rbx #x-8) rax)
2f: 48b8a80e484c00000000 (mov rax #x4c480ea8)
39: 488b00 (mov rax (mem64+ rax))
3c: 488983f0ffffff (mov (mem64+ rbx #x-10) rax)
43: 4881c3f0ffffff (add rbx #xfffffffffffffff0)
4a: 488b4308 (mov rax (mem64+ rbx #x8))
4e: f6c001 (test al #x1)
51: 0f850f000000 (jnz (+ rip #xf))
57: 66833831 (cmp (mem16+ rax) #x31)
5b: 0f8505000000 (jnz (+ rip #x5))
61: e81e771fb4 (call (+ rip #x-4be088e2))
66: e8bb751fb4 (call (+ rip #x-4be08a45))
6b: 488b4320 (mov rax (mem64+ rbx #x20))
6f: 4883c310 (add rbx #x10)
73: f6c001 (test al #x1)
76: 0f8514000000 (jnz (+ rip #x14))
7c: ba60fba070 (mov edx #x70a0fb60)
81: b9ff010000 (mov ecx #x1ff)
86: e86e001fb4 (call (+ rip #x-4be0ff92))
8b: e907000000 (jmp (+ rip #x7))
90: 4881e0ff010000 (and rax #x1ff)
97: 4c8b75c8 (mov r14 (mem64+ rbp #x-38))
9b: 4883c428 (add rsp #x28)
9f: 5f (pop rdi)
a0: 5e (pop rsi)
a1: 5b (pop rbx)
a2: 5d (pop rbp)
a3: c3 (ret)
(define (unsafe-lsb x)
(unsafe-fxand x #xff))
(disassemble unsafe-lsb)
0: 488983f8ffffff (mov (mem64+ rbx #x-8) rax)
7: 4881c3f8ffffff (add rbx #xfffffffffffffff8)
e: 488b4308 (mov rax (mem64+ rbx #x8))
12: 4881e0ff010000 (and rax #x1ff)
19: 4c8b75c8 (mov r14 (mem64+ rbp #x-38))
1d: 4883c428 (add rsp #x28)
21: 5f (pop rdi)
22: 5e (pop rsi)
23: 5b (pop rbx)
24: 5d (pop rbp)
25: c3 (ret)
This returns two assembly listings and as expected the unsafe-lsb is
shorter than the one for lsb but not even close to the one you'd get in
C. Is there something else in Racket that I might be missing that it
gives me access to the low level C bit manip constructs besides ffi?
Kind regards,
--
Paulo Matos
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