3 ms·

I've never seen anyone store 10 byte IEEE-754 values in memory. People store doubles, and people get upset when compiler optimizations (like when to spill from

• qb45 10y ago
  > I've never seen anyone store 10 byte IEEE-754 values in memory. But you can, if you are after precision as the OP apparently was. I'm not sure what this c-word is doing in your post, I thought we were talking assembly, but as far as c-things go, at least gcc and clang represent long double as 80b extended precision on x86. So, for example, compiling this beauty (which is too large for x87 stack): long double a[64], b[64], c[64], d[64]; // load a,b,c,d from somewhere long double x = ((((((((a[0]+a[1])+(a[2]+a[3]))+((a[4]+a[5])+(a[6]+a[7]))) +(((a[8]+a[9])+(a[10]+a[11]))+((a[12]+a[13])+(a[14]+a[15])))) +((((a[16]+a[17])+(a[18]+a[19]))+((a[20]+a[21])+(a[22]+a[23]))) +(((a[24]+a[25])+(a[26]+a[27]))+((a[28]+a[29])+(a[30]+a[31]))))) +(((((a[32]+a[33])+(a[34]+a[35]))+((a[36]+a[37])+(a[38]+a[39]))) +(((a[40]+a[41])+(a[42]+a[43]))+((a[44]+a[45])+(a[46]+a[47])))) +((((a[48]+a[49])+(a[50]+a[51]))+((a[52]+a[53])+(a[54]+a[55]))) +(((a[56]+a[57])+(a[58]+a[59]))+((a[60]+a[61])+(a[62]+a[63])))))) +((((((b[0]+b[1])+(b[2]+b[3]))+((b[4]+b[5])+(b[6]+b[7]))) +(((b[8]+b[9])+(b[10]+b[11]))+((b[12]+b[13])+(b[14]+b[15])))) +((((b[16]+b[17])+(b[18]+b[19]))+((b[20]+b[21])+(b[22]+b[23]))) +(((b[24]+b[25])+(b[26]+b[27]))+((b[28]+b[29])+(b[30]+b[31]))))) +(((((b[32]+b[33])+(b[34]+b[35]))+((b[36]+b[37])+(b[38]+b[39]))) +(((b[40]+b[41])+(b[42]+b[43]))+((b[44]+b[45])+(b[46]+b[47])))) +((((b[48]+b[49])+(b[50]+b[51]))+((b[52]+b[53])+(b[54]+b[55]))) +(((b[56]+b[57])+(b[58]+b[59]))+((b[60]+b[61])+(b[62]+b[63]))))))) +(((((((c[0]+c[1])+(c[2]+c[3]))+((c[4]+c[5])+(c[6]+c[7]))) +(((c[8]+c[9])+(c[10]+c[11]))+((c[12]+c[13])+(c[14]+c[15])))) +((((c[16]+c[17])+(c[18]+c[19]))+((c[20]+c[21])+(c[22]+c[23]))) +(((c[24]+c[25])+(c[26]+c[27]))+((c[28]+c[29])+(c[30]+c[31]))))) +(((((c[32]+c[33])+(c[34]+c[35]))+((c[36]+c[37])+(c[38]+c[39]))) +(((c[40]+c[41])+(c[42]+c[43]))+((c[44]+c[45])+(c[46]+c[47])))) +((((c[48]+c[49])+(c[50]+c[51]))+((c[52]+c[53])+(c[54]+c[55]))) +(((c[56]+c[57])+(c[58]+c[59]))+((c[60]+c[61])+(c[62]+c[63])))))) +((((((d[0]+d[1])+(d[2]+d[3]))+((d[4]+d[5])+(d[6]+d[7]))) +(((d[8]+d[9])+(d[10]+d[11]))+((d[12]+d[13])+(d[14]+d[15])))) +((((d[16]+d[17])+(d[18]+d[19]))+((d[20]+d[21])+(d[22]+d[23]))) +(((d[24]+d[25])+(d[26]+d[27]))+((d[28]+d[29])+(d[30]+d[31]))))) +(((((d[32]+d[33])+(d[34]+d[35]))+((d[36]+d[37])+(d[38]+d[39]))) +(((d[40]+d[41])+(d[42]+d[43]))+((d[44]+d[45])+(d[46]+d[47])))) +((((d[48]+d[49])+(d[50]+d[51]))+((d[52]+d[53])+(d[54]+d[55]))) +(((d[56]+d[57])+(d[58]+d[59]))+((d[60]+d[61])+(d[62]+d[63])))))))) ; produces only 80b spills (fstpt in GNU syntax): $ objdump -d fpmonster |grep fst 400457: db 7c 1c 10 fstpt 0x10(%rsp,%rbx,1) 400462: db bc 1c 10 04 00 00 fstpt 0x410(%rsp,%rbx,1) 400470: db bc 1c 10 08 00 00 fstpt 0x810(%rsp,%rbx,1) 40047e: db bc 1c 10 0c 00 00 fstpt 0xc10(%rsp,%rbx,1) 400b47: db 7c 24 10 fstpt 0x10(%rsp) 400d91: db 3c 24 fstpt (%rsp) Clearly, extended precision can be done right both in C and raw assembly. > People store doubles, and people get upset That's their fault :) and another story altogether. For reproducible low precision, indeed SSE is the way to go.