restrict keyword — aliasing hint — Hinglish notes

5.1.32 · Coding › C Programming

WHAT is `restrict`?

Aliasing = do alag pointers ek hi memory ko refer karte hain (overlap karte hain).
restrict ek anti-aliasing hint hai: yeh assert karta hai ki us pointer ke through koi aliasing nahi hai.
Promise todna → undefined behaviour. Compiler aap par blindly trust karta hai.

WHY does it exist?

Derivation-from-scratch: cost kahan se aata hai

Socho:

void add(int *a, int *b, int *c, int n) {
    for (int i = 0; i < n; i++)
        a[i] = b[i] + c[i];
}

Compiler nahi jaanta ki a, b ya c ko overlap karta hai ya nahi. Agar a == b+1 ho, to a[i] mein likhne se b[i+1] badal jaata hai. To compiler majboor hai:

b[i] load karo
c[i] load karo
a[i] mein store karo …strictly usi order mein, har iteration, koi vectorization nahi, kyunki ek store future load ko invalidate kar sakta hai.

Ab inhe mark karo:

void add(int * restrict a, const int * restrict b,
         const int * restrict c, int n) {
    for (int i = 0; i < n; i++)
        a[i] = b[i] + c[i];
}

Tumne promise kiya ki a, b, c overlap nahi karte. Ab compiler:

b aur c ka ek block prefetch kar sakta hai,
kai sums ek saath compute kar sakta hai (SIMD),
a ka block store kar sakta hai — order ab matter nahi karta.

Figure — restrict keyword — aliasing hint

HOW to use it

Worked example Example 1 — safe copy

void copy(int * restrict dst, const int * restrict src, int n) {
    for (int i = 0; i < n; i++) dst[i] = src[i];
}

Yeh step kyun? Hum promise karte hain ki dst aur src regions overlap nahi karte, to compiler kisi bhi order mein copy kar sakta hai / vectorize kar sakta hai. Isliye memcpy (jo restrict semantics use karta hai) overlapping regions ke liye undefined hai, jabki memmove (aisa koi promise nahi) safe hai.

Worked example Example 2 — jhooth bolne ka khatre

int arr[4] = {1,2,3,4};
copy(arr + 1, arr, 3);   // dst and src OVERLAP -> UB!

Yeh step kyun? Yahan dst = arr+1, src = arr. Yeh overlap karte hain, lekin humne restrict version use kiya. Output undefined hai — shayad {1,1,2,3} ho, shayad {1,1,1,1}. Iske bajaye memmove semantics (bina restrict ke) use karo.

Worked example Example 3 — single pointer self-promise

void scale(float * restrict v, float * restrict factor, int n) {
    for (int i = 0; i < n; i++) v[i] *= *factor;
}

Yeh step kyun? Bina restrict ke, factor v ke andar point kar sakta hai, to compiler har iteration mein *factor dobara load karta. restrict factor ke saath, woh *factor ek baar register mein load karta hai. Badi speedup milti hai.

Common mistakes (Steel-man)

Recall Feynman: 12-saal ke bachche ko samjhao

Do robots hain, dono ko numbers likhne ki list di gayi hai. Agar tum unhe batao ki "tumhare dono notebooks bilkul alag hain," woh bina ek doosre ko check kiye super fast likh sakte hain. Lekin agar tum chupke se unhe ek hi notebook do aur claim karo ki woh alag hain, woh ek doosre ke upar scribble karte hain aur tumhe garbage milta hai. restrict matlab tum robots se promise kar rahe ho ki unki notebooks alag hain — aur agar tum jhooth bolte ho, result bakwaas hoga.

Active recall

Recall Quick self-test

restrict kis category ka token hai? (qualifier? keyword?)
Kya compiler tumhara restrict promise verify karta hai?
Kya hota hai agar do restrict pointers actually overlap karein?
Kaun sa standard function restrict-like semantics rely karta hai: memcpy ya memmove?

C mein restrict kis tarah ka token hai?

Ek type qualifier (C99) jo pointers pe lagaya jaata hai, jaise const ek qualifier hai.

T * restrict p kya promise karta hai?

Ki p ki lifetime mein, pointed-to object sirf p ke through (ya p se derived pointers ke through) access hota hai — yaani koi aliasing nahi.

Kya compiler restrict promise verify karta hai?

Nahi. Woh trust karta hai; ise todna undefined behaviour hai.

Restrict faster code kyun enable karta hai?

Yeh compiler ki worst-case assumption hata deta hai ki pointers alias kar sakte hain, jisse reordering, values ko registers mein cache karna, aur vectorization possible hoti hai.

Kya hota hai agar do restrict pointers actually overlap karein?

Undefined behaviour — results galat ho sakte hain, aksar sirf higher optimization levels pe.

Kya restrict modifiability ke baare mein hai?

Nahi, woh const ka kaam hai. Restrict purely ek aliasing (non-overlap) hint hai.

Kaun sa function restrict-style 'no overlap' semantics rakhta hai, memcpy ya memmove?

memcpy (overlap UB hai); memmove explicitly overlap allow karta hai.

for(i) v[i]*=*factor; mein factor ko restrict kyun mark karein?

Taaki compiler jaane ki factor v ke andar point nahi karta, aur woh *factor ko ek baar register mein load kar sake, har iteration mein dobara padhne ke bajaye.

R.E.S.T. mnemonic ka kya matlab hai?

Regions Entirely Separate, Trust me.

Connections

Pointers in C
const qualifier
Pointer aliasing
memcpy vs memmove
Compiler optimization & vectorization (SIMD)
Undefined behaviour in C
C99 standard features