[llvm-dev] Clang -Oz vs Opt -Oz

Tue Jan 19 11:45:26 PST 2021

Hi All

I am trying to analyze the difference between binaries produced using clang
-Oz and Opt -Oz. All the binaries produced are using the -c flag. Right off
the bat, I can see the difference between the IR file produced by clang -Oz
which adds "minsize","optsize" and removes the "no-inline" function
attribute. To keep things similar the same is done to the default IR
produced for the following code with the optnone attribute removed using
the -disable-O0-optnone flag.

#include <stdio.h>
int num[326] = {
 1,
 2,
 3,
 4,
 6,
 8,
 9,
 12,
 16,
 18,
 24,
 27,
 32,
 36,
 48,
 54,
 64,
 72,
 81,
 96,
 108,
 128,
 144,
 162,
 192,
 216,
 243,
 256,
 288,
 324,
 384,
 432,
 486,
 512,
 576,
 648,
 729,
 768,
 864,
 972,
 1024,
 1152,
 1296,
 1458,
 1536,
 1728,
 1944,
 2048,
 2187,
 2304,
 2592,
 2916,
 3072,
 3456,
 3888,
 4096,
 4374,
 4608,
 5184,
 5832,
 6144,
 6561,
 6912,
 7776,
 8192,
 8748,
 9216,
 10368,
 11664,
 12288,
 13122,
 13824,
 15552,
 16384,
 17496,
 18432,
 19683,
 20736,
 23328,
 24576,
 26244,
 27648,
 31104,
 32768,
 34992,
 36864,
 39366,
 41472,
 46656,
 49152,
 52488,
 55296,
 59049,
 62208,
 65536,
 69984,
 73728,
 78732,
 82944,
 93312,
 98304,
 104976,
 110592,
 118098,
 124416,
 131072,
 139968,
 147456,
 157464,
 165888,
 177147,
 186624,
 196608,
 209952,
 221184,
 236196,
 248832,
 262144,
 279936,
 294912,
 314928,
 331776,
 354294,
 373248,
 393216,
 419904,
 442368,
 472392,
 497664,
 524288,
 531441,
 559872,
 589824,
 629856,
 663552,
 708588,
 746496,
 786432,
 839808,
 884736,
 944784,
 995328,
 1048576,
 1062882,
 1119744,
 1179648,
 1259712,
 1327104,
 1417176,
 1492992,
 1572864,
 1594323,
 1679616,
 1769472,
 1889568,
 1990656,
 2097152,
 2125764,
 2239488,
 2359296,
 2519424,
 2654208,
 2834352,
 2985984,
 3145728,
 3188646,
 3359232,
 3538944,
 3779136,
 3981312,
 4194304,
 4251528,
 4478976,
 4718592,
 4782969,
 5038848,
 5308416,
 5668704,
 5971968,
 6291456,
 6377292,
 6718464,
 7077888,
 7558272,
 7962624,
 8388608,
 8503056,
 8957952,
 9437184,
 9565938,
 10077696,
 10616832,
 11337408,
 11943936,
 12582912,
 12754584,
 13436928,
 14155776,
 14348907,
 15116544,
 15925248,
 16777216,
 17006112,
 17915904,
 18874368,
 19131876,
 20155392,
 21233664,
 22674816,
 23887872,
 25165824,
 25509168,
 26873856,
 28311552,
 28697814,
 30233088,
 31850496,
 33554432,
 34012224,
 35831808,
 37748736,
 38263752,
 40310784,
 42467328,
 43046721,
 45349632,
 47775744,
 50331648,
 51018336,
 53747712,
 56623104,
 57395628,
 60466176,
 63700992,
 67108864,
 68024448,
 71663616,
 75497472,
 76527504,
 80621568,
 84934656,
 86093442,
 90699264,
 95551488,
 100663296,
 102036672,
 107495424,
 113246208,
 114791256,
 120932352,
 127401984,
 129140163,
 134217728,
 136048896,
 143327232,
 150994944,
 153055008,
 161243136,
 169869312,
 172186884,
 181398528,
 191102976,
 201326592,
 204073344,
 214990848,
 226492416,
 229582512,
 241864704,
 254803968,
 258280326,
 268435456,
 272097792,
 286654464,
 301989888,
 306110016,
 322486272,
 339738624,
 344373768,
 362797056,
 382205952,
 387420489,
 402653184,
 408146688,
 429981696,
 452984832,
 459165024,
 483729408,
 509607936,
 516560652,
 536870912,
 544195584,
 573308928,
 603979776,
 612220032,
 644972544,
 679477248,
 688747536,
 725594112,
 764411904,
 774840978,
 805306368,
 816293376,
 859963392,
 905969664,
 918330048,
 967458816,
 1019215872,
 1033121304,
 1073741824,
 1088391168,
 1146617856,
 1162261467,
 1207959552,
 1224440064,
 1289945088,
 1358954496,
 1377495072,
 1451188224,
 1528823808,
 1549681956,
 1610612736,
 1632586752,
 1719926784,
 1811939328,
 1836660096,
 1934917632
};
int main() {
 int l, r;
    scanf("%d %d", &l, &r);

 int L = -1, R = 326;
 for (int i = 0; i < 326; ++i) {
 if (num[i] < l) {
            L = i;
 }
 }
 for (int i = 325; i >= 0; --i) {
 if (num[i] > r) {
            R = i;
 }
 }
    printf("%d", (R - L - (long long) 1));
 return 0;
}

What follows next is applying opt -S -Oz on the same IR followed by clang
-c <Oz_IR_File> -o  binname.out to create the binary

The binary thus formed has a size of 3072 bytes whereas the binary formed
with Clang -Oz on the c file viz clang  -c -Oz filename.c has a size of
2944 bytes and the most interesting part is that the out file produced
without any optimization viz clang -c  filename.c is 3040 bytes

Are there some other attributes that need to be added to get Opt -Oz
similar to clang -Oz or am I missing something?

P.S: Apologies for the huge chunk of code in the middle of the mail. I was
not sure if attaching it would be viewable or not.

P.S.S: This is one file out of a few more files that result in the
discrepancy.

Regards
Anilava

-- 

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