| Author: | Wojciech Muła |
|---|
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Table of contents
With simple program following times were measured:
Function defined in scched.h returns the number of a CPU that runs calling thread or process.
There is no such instruction — CVTDQ2PS converts signed 32-bit ints. Solution: first zero the MSB, such number is never negative in U2, so mentioned instruction could be used. Then add 232 if the MSB was set.
float CONST[4] SIMD_ALIGN = packed_float((float)((uint32_t)(1 << 31))); /* 2^31 */ uint32_t MASK_0_30[4] SIMD_ALIGN = packed_dword(0x7fffffff); uint32_t MASK_31[4] SIMD_ALIGN = packed_dword(0x80000000); void convert_uint32_float(uint32_t in[4], float out[4]) { __asm__ volatile ( "movdqu (%%eax), %%xmm0 \n" "movdqa %%xmm0, %%xmm1 \n" "pand MASK_0_30, %%xmm0 \n" // xmm0 - mask MSB bit - never less then zero in U2 "cvtdq2ps %%xmm0, %%xmm0 \n" // convert this value to float "psrad $32, %%xmm1 \n" // populate MSB in higher word (enough to mask CONST) "pand CONST, %%xmm1 \n" // xmm1 = MSB set ? float(2^31) : float(0) "addps %%xmm1, %%xmm0 \n" // add 2^31 if MSB set "movdqu %%xmm0, (%%ebx) \n" : /* no output */ : "a" (in), "b" (out) ); }
Branch-less x86 code:
movl %eax, %ebx sarl $32, %ebx ; fill ebx with sign bit xorl %ebx, %eax ; negate eax (if negative) subl %ebx, %eax ; increment eax by 1 (if negative)
SSE2:
pshufd $0b11110101, %xmm0, %xmm1 ; populate dwords 3 and 1 psrad $32, %xmm1 ; fill quad words with sign bit pxor %xmm1, %xmm0 ; negate (if negative) psubq %xmm1, %xmm0 ; increment (if negative)
RDTSC is incremented with bus-clock cycles, and then multiplied by core-clock/bus-clock ratio. From programmer view, RDTSC counter is incremented by value greater then 1, for example on C2D E8200 it is 8.
Latency of RDTSC in Pentium4 is about 60-120 cycles, on AMD CPU around 6 cycles.
asm(
"..."
: "+a" (var)
);
This won't work, GCC complains:
asm(
"..."
: /* no output */
: "a" (var)
: "eax"
);
We can declare a temporary variable, and treat it as read-write:
int tmp_var = var;
asm(
"..."
: "+a" (tmp_var)
);
If there are more registers, or var shouldn't be changed, then we can declare a common dummy variable:
int dummy __attribute__((unused));
asm(
"..."
: "=a" (dummy),
"=b" (dummy),
"=c" (dummy)
: "a" (var_or_value1),
"b" (var_or_value2),
"c" (var_or_value2)
);