Hi,
targeting AMD64 alias x86_64 with -O3, GCC 10.2.0 generates the
following code (19 instructions using 86 bytes, plus 6 quadwords
using 48 bytes) for __builtin_floor() when -msse4.1 is NOT given:
.text
0: f2 0f 10 15 10 00 00 00 movsd .LC1(%rip), %xmm2
4: R_X86_64_PC32 .rdata
8: f2 0f 10 25 00 00 00 00 movsd .LC0(%rip), %xmm4
c: R_X86_64_PC32 .rdata
10: 66 0f 28 d8 movapd %xmm0, %xmm3
14: 66 0f 28 c8 movapd %xmm0, %xmm1
18: 66 0f 54 da andpd %xmm2, %xmm3
1c: 66 0f 2e e3 ucomisd %xmm3, %xmm4
20: 76 33 jbe 55 <_floor+0x55>
22: f2 48 0f 2c c0 cvttsd2si %xmm0, %rax
27: 66 0f ef db pxor %xmm3, %xmm3
2b: 66 0f 55 d1 andnpd %xmm1, %xmm2
2f: f2 48 0f 2a d8 cvtsi2sd %rax, %xmm3
34: 66 0f 28 e3 movapd %xmm3, %xmm4
38: f2 0f c2 e0 06 cmpnlesd %xmm0, %xmm4
3d: f2 0f 10 05 20 00 00 00 movsd .LC2(%rip), %xmm0
91: R_X86_64_PC32 .rdata
45: 66 0f 54 e0 andpd %xmm0, %xmm4
49: f2 0f 5c dc subsd %xmm4, %xmm3
4d: 66 0f 28 c3 movapd %xmm3, %xmm0
51: 66 0f 56 c2 orpd %xmm2, %xmm0
55: c3 retq
.rdata
.align 8
0: 00 00 00 00 .LC0: .quad 0x1.0p52
00 00 30 43
00 00 00 00
00 00 00 00
.align 16
10: ff ff ff ff .LC1: .quad ~(-0.0)
ff ff ff 7f
18: 00 00 00 00 .quad 0.0
00 00 00 00
.align 8
20: 00 00 00 00 .LC2: .quad 0x1.0p0
00 00 f0 3f
00 00 00 00
00 00 00 00
.end
JFTR: in the best case, the memory accesses cost several cycles,
while in the worst case they yield a page fault!
Properly optimized, shorter and faster code, using only 15 instructions
in just 65 bytes, WITHOUT superfluous constants, thus avoiding costly
memory accesses and saving at least 16 + 48 bytes, follows:
.intel_syntax
.equ BIAS, 1023
.text
0: f2 48 0f 2c c0 cvttsd2si rax, xmm0 # rax = trunc(argument)
5: 48 f7 d8 neg rax
# jz .L0 # argument zero?
8: 70 36 jo .L0 # argument indefinite?
# argument overflows 64-bit
integer?
a: 48 f7 d8 neg rax
d: f2 48 0f 2a c8 cvtsi2sd xmm1, rax # xmm1 = trunc(argument)
12: 48 a1 00 00 00 mov rax, (1 << 63) | (BIAS << 52)
19: 00 00 00 f0 bf
1c: 66 48 0f 6e d0 movq xmm2, rax # xmm2 = -0x1.0p0
21: f2 0f 10 d8 movsd xmm3, xmm0 # xmm3 = argument
25: f2 0f c2 d9 01 cmpltsd xmm3, xmm1 # xmm3 = (argument <
trunc(argument)) ? ~0L : 0L
2a: 66 0f 54 da andpd xmm3, xmm2 # xmm3 = (argument <
trunc(argument)) ? -1.0 : 0.0
2e: f2 0f 58 d9 addsd xmm3, xmm1 # xmm3 = (argument <
trunc(argument)) ? -1.0 : 0.0
# + trunc(argument)
# = floor(argument)
32: 66 0f 73 d0 3f psrlq xmm0, 63
37: 66 0f 73 f0 3f psllq xmm0, 63 # xmm0 = (argument & -0.0) ?
-0.0 : 0.0
3c: 66 0f 56 c3 orpd xmm0, xmm3 # xmm0 = floor(argument)
40: c3 .L0: ret
.end
regards
Stefan
