Hi,
targeting AMD64 alias x86_64 with -O3, GCC 10.2.0 generates the
following code (13 instructions using 57 bytes, plus 4 quadwords
using 32 bytes) for __builtin_trunc() 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 16 jbe 38 <_trunc+0x38>
22: f2 48 0f 2c c0 cvttsd2si %xmm0, %rax
27: 66 0f ef c0 pxor %xmm0, %xmm0
2b: 66 0f 55 d1 andnpd %xmm1, %xmm2
2f: f2 48 0f 2a c0 cvtsi2sd %rax, %xmm0
34: 66 0f 56 c2 orpd %xmm2, %xmm0
38: 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
.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 but only 9 instructions
in just 33 bytes, WITHOUT any constants, thus avoiding costly memory accesses
and saving at least 16 + 32 bytes, follows:
.intel_syntax
.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 16 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: 66 0f 73 d0 3f psrlq xmm0, 63
17: 66 0f 73 f0 3f psllq xmm0, 63 # xmm0 = (argument & -0.0) ?
-0.0 : 0.0
1c: 66 0f 56 c1 orpd xmm0, xmm1 # xmm0 = trunc(argument)
20: c3 .L0: ret
.end
regards
Stefan
