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tls: P256: improve x86_64 multiplication asm code 

gcc is being rather silly. Usues suboptimal registers,
and does not realize that i and j are never negative,
thus usese even _more_ registers for temporaries
to sign-extend i/j to 64-bit offsets.

function                                             old     new   delta
sp_256_mont_mul_8                                    155     132     -23

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
master
Denys Vlasenko 2024-07-12 11:08:08 +02:00
parent d745852f13
commit 999e290ef6
1 changed files with 36 additions and 22 deletions
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58
networking/tls_sp_c32.c
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@ -411,10 +411,10 @@ static void sp_256_sub_8_p256_mod(sp_digit* r)
"\n subl $0xffffffff, (%0)"
"\n sbbl $0xffffffff, 1*4(%0)"
"\n sbbl $0xffffffff, 2*4(%0)"
"\n sbbl $0, 3*4(%0)"
"\n sbbl $0, 4*4(%0)"
"\n sbbl $0, 5*4(%0)"
"\n sbbl $1, 6*4(%0)"
"\n sbbl $0x00000000, 3*4(%0)"
"\n sbbl $0x00000000, 4*4(%0)"
"\n sbbl $0x00000000, 5*4(%0)"
"\n sbbl $0x00000001, 6*4(%0)"
"\n sbbl $0xffffffff, 7*4(%0)"
"\n"
: "=r" (r)
@ -433,10 +433,10 @@ static void sp_256_sub_8_p256_mod(sp_digit* r)
uint64_t ooff;
asm volatile (
"\n subq $0xffffffffffffffff, (%0)"
"\n sbbq %1, 1*8(%0)"
"\n sbbq $0, 2*8(%0)"
"\n sbbq %1, 1*8(%0)" // %1 = 00000000ffffffff
"\n sbbq $0x0000000000000000, 2*8(%0)"
"\n movq 3*8(%0), %2"
"\n sbbq $0, %2" // subtract carry
"\n sbbq $0x0, %2" // subtract carry
"\n addq %1, %2" // adding 00000000ffffffff (in %1)
"\n" // is the same as subtracting ffffffff00000001
"\n movq %2, 3*8(%0)"
@ -452,9 +452,9 @@ static void sp_256_sub_8_p256_mod(sp_digit* r)
"\n orl $0xffffffff, %%eax" // %1 (rax) = 00000000ffffffff
"\n subq $0xffffffffffffffff, (%0)"
"\n sbbq %1, 1*8(%0)"
"\n sbbq $0, 2*8(%0)"
"\n sbbq $0x0000000000000000, 2*8(%0)"
"\n movq 3*8(%0), %2"
"\n sbbq $0, %2" // subtract carry
"\n sbbq $0x0, %2" // subtract carry
"\n addq %1, %2" // adding 00000000ffffffff (in %1)
"\n" // is the same as subtracting ffffffff00000001
"\n movq %2, 3*8(%0)"
@ -495,15 +495,23 @@ static void sp_256to512_mul_8(sp_digit* r, const sp_digit* a, const sp_digit* b)
////////////////////////
// uint64_t m = ((uint64_t)a[i]) * b[j];
// acc_hi:acch:accl += m;
long eax_clobbered;
asm volatile (
// a[i] is already loaded in %%eax
"\n mull %7"
"\n mull %8"
"\n addl %%eax, %0"
"\n adcl %%edx, %1"
"\n adcl $0, %2"
: "=rm" (accl), "=rm" (acch), "=rm" (acc_hi)
: "0" (accl), "1" (acch), "2" (acc_hi), "a" (a[i]), "m" (b[j])
"\n adcl $0x0, %2"
: "=rm" (accl), "=rm" (acch), "=rm" (acc_hi), "=a" (eax_clobbered)
: "0" (accl), "1" (acch), "2" (acc_hi), "3" (a[i]), "m" (b[j])
: "cc", "dx"
// What is "eax_clobbered"? gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html:
// "Do not modify the contents of input-only operands (except for inputs tied
// to outputs). The compiler assumes that on exit from the asm statement these
// operands contain the same values as they had before executing the statement.
// It is not possible to use clobbers to inform the compiler that the values
// in these inputs are changing. One common work-around is to tie the changing
// input variable to an output variable that never gets used."
);
////////////////////////
j--;
@ -519,15 +527,20 @@ static void sp_256to512_mul_8(sp_digit* r, const sp_digit* a, const sp_digit* b)
const uint64_t* bb = (const void*)b;
uint64_t* rr = (void*)r;
int k;
uint64_t accl;
uint64_t acch;
register uint64_t accl asm("r8");
register uint64_t acch asm("r9");
/* ^^^ ask gcc to not use rax/rdx/input arg regs for accumulator variables */
/* (or else it may generate lots of silly mov's and even xchg's!) */
acch = accl = 0;
for (k = 0; k < 7; k++) {
int i, j;
uint64_t acc_hi;
unsigned i, j;
/* ^^^^^ not signed "int",
* or gcc can use a temp register to sign-extend i,j for aa[i],bb[j] */
register uint64_t acc_hi asm("r10");
/* ^^^ ask gcc to not use rax/rdx/input arg regs for accumulators */
i = k - 3;
if (i < 0)
if ((int)i < 0)
i = 0;
j = k - i;
acc_hi = 0;
@ -535,14 +548,15 @@ static void sp_256to512_mul_8(sp_digit* r, const sp_digit* a, const sp_digit* b)
////////////////////////
// uint128_t m = ((uint128_t)a[i]) * b[j];
// acc_hi:acch:accl += m;
long rax_clobbered;
asm volatile (
// aa[i] is already loaded in %%rax
"\n mulq %7"
"\n mulq %8"
"\n addq %%rax, %0"
"\n adcq %%rdx, %1"
"\n adcq $0, %2"
: "=rm" (accl), "=rm" (acch), "=rm" (acc_hi)
: "0" (accl), "1" (acch), "2" (acc_hi), "a" (aa[i]), "m" (bb[j])
"\n adcq $0x0, %2"
: "=rm" (accl), "=rm" (acch), "=rm" (acc_hi), "=a" (rax_clobbered)
: "0" (accl), "1" (acch), "2" (acc_hi), "3" (aa[i]), "m" (bb[j])
: "cc", "dx"
);
////////////////////////