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git/block-sha1/sha1.c
brian m. carlson 544d93bc3b block-sha1: remove use of obsolete x86 assembly 
In the block SHA-1 code, we have special assembly code for i386 and
amd64 to perform rotations with assembly.  This is supposed to help pick
the correct rotation operation depending on which rotation is smaller,
which can help some systems perform slightly better, since any circular
rotation can be specified as either a rotate left or a rotate right.
However, this isn't needed, so we should remove it.

First, SHA-1, like SHA-2, uses fixed constant rotates.  Thus, all
rotation amounts are known at compile time and are in fact baked into
the code.  Fortunately, peephole optimizers recognize rotations
specified in the normal way and automatically emit the correct code,
including a preference for choosing a rotate left versus a rotate right.
This has been the case for well over a decade, and is a standard example
of the utility of a peephole optimizer.

Moreover, all modern CPUs, with the exception of extremely limited
embedded CPUs such as some Cortex-M processors, provide a barrel
shifter, which lets the CPU perform rotates of any bit amount in
constant time.  This is valuable for many cryptographic algorithms to
improve performance, and is required to prevent timing attacks in
algorithms which use data-dependent rotations (which don't include the
hash algorithms we use).  As a result, even though the compiler does the
correct optimization, it isn't even needed here and either a left or a
right rotate is equally acceptable.

In fact, the SHA-256 code already takes this into account and just
writes the simple code using an inline function to let the compiler
optimize it for us.

The downside of using this code, however, is that it uses a GCC
extension, which makes the compiler complain when using -pedantic unless
it's prefixed with __extension__.  We could fix that, but since it's
not needed, let's just remove it.  We haven't noticed this because
almost everyone uses the SHA1DC code instead, but it still shows up for
some people.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2022-03-10 11:18:05 -08:00

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/*
* SHA1 routine optimized to do word accesses rather than byte accesses,
* and to avoid unnecessary copies into the context array.
*
* This was initially based on the Mozilla SHA1 implementation, although
* none of the original Mozilla code remains.
*/
/* this is only to get definitions for memcpy(), ntohl() and htonl() */
#include "../git-compat-util.h"
#include "sha1.h"
#define SHA_ROT(X,l,r) (((X) << (l)) | ((X) >> (r)))
#define SHA_ROL(X,n) SHA_ROT(X,n,32-(n))
#define SHA_ROR(X,n) SHA_ROT(X,32-(n),n)
/*
* If you have 32 registers or more, the compiler can (and should)
* try to change the array[] accesses into registers. However, on
* machines with less than ~25 registers, that won't really work,
* and at least gcc will make an unholy mess of it.
*
* So to avoid that mess which just slows things down, we force
* the stores to memory to actually happen (we might be better off
* with a 'W(t)=(val);asm("":"+m" (W(t))' there instead, as
* suggested by Artur Skawina - that will also make gcc unable to
* try to do the silly "optimize away loads" part because it won't
* see what the value will be).
*
* Ben Herrenschmidt reports that on PPC, the C version comes close
* to the optimized asm with this (ie on PPC you don't want that
* 'volatile', since there are lots of registers).
*
* On ARM we get the best code generation by forcing a full memory barrier
* between each SHA_ROUND, otherwise gcc happily get wild with spilling and
* the stack frame size simply explode and performance goes down the drain.
*/
#if defined(__i386__) || defined(__x86_64__)
#define setW(x, val) (*(volatile unsigned int *)&W(x) = (val))
#elif defined(__GNUC__) && defined(__arm__)
#define setW(x, val) do { W(x) = (val); __asm__("":::"memory"); } while (0)
#else
#define setW(x, val) (W(x) = (val))
#endif
/* This "rolls" over the 512-bit array */
#define W(x) (array[(x)&15])
/*
* Where do we get the source from? The first 16 iterations get it from
* the input data, the next mix it from the 512-bit array.
*/
#define SHA_SRC(t) get_be32((unsigned char *) block + (t)*4)
#define SHA_MIX(t) SHA_ROL(W((t)+13) ^ W((t)+8) ^ W((t)+2) ^ W(t), 1)
#define SHA_ROUND(t, input, fn, constant, A, B, C, D, E) do { \
unsigned int TEMP = input(t); setW(t, TEMP); \
E += TEMP + SHA_ROL(A,5) + (fn) + (constant); \
B = SHA_ROR(B, 2); } while (0)
#define T_0_15(t, A, B, C, D, E) SHA_ROUND(t, SHA_SRC, (((C^D)&B)^D) , 0x5a827999, A, B, C, D, E )
#define T_16_19(t, A, B, C, D, E) SHA_ROUND(t, SHA_MIX, (((C^D)&B)^D) , 0x5a827999, A, B, C, D, E )
#define T_20_39(t, A, B, C, D, E) SHA_ROUND(t, SHA_MIX, (B^C^D) , 0x6ed9eba1, A, B, C, D, E )
#define T_40_59(t, A, B, C, D, E) SHA_ROUND(t, SHA_MIX, ((B&C)+(D&(B^C))) , 0x8f1bbcdc, A, B, C, D, E )
#define T_60_79(t, A, B, C, D, E) SHA_ROUND(t, SHA_MIX, (B^C^D) , 0xca62c1d6, A, B, C, D, E )
static void blk_SHA1_Block(blk_SHA_CTX *ctx, const void *block)
{
unsigned int A,B,C,D,E;
unsigned int array[16];
A = ctx->H[0];
B = ctx->H[1];
C = ctx->H[2];
D = ctx->H[3];
E = ctx->H[4];
/* Round 1 - iterations 0-16 take their input from 'block' */
T_0_15( 0, A, B, C, D, E);
T_0_15( 1, E, A, B, C, D);
T_0_15( 2, D, E, A, B, C);
T_0_15( 3, C, D, E, A, B);
T_0_15( 4, B, C, D, E, A);
T_0_15( 5, A, B, C, D, E);
T_0_15( 6, E, A, B, C, D);
T_0_15( 7, D, E, A, B, C);
T_0_15( 8, C, D, E, A, B);
T_0_15( 9, B, C, D, E, A);
T_0_15(10, A, B, C, D, E);
T_0_15(11, E, A, B, C, D);
T_0_15(12, D, E, A, B, C);
T_0_15(13, C, D, E, A, B);
T_0_15(14, B, C, D, E, A);
T_0_15(15, A, B, C, D, E);
/* Round 1 - tail. Input from 512-bit mixing array */
T_16_19(16, E, A, B, C, D);
T_16_19(17, D, E, A, B, C);
T_16_19(18, C, D, E, A, B);
T_16_19(19, B, C, D, E, A);
/* Round 2 */
T_20_39(20, A, B, C, D, E);
T_20_39(21, E, A, B, C, D);
T_20_39(22, D, E, A, B, C);
T_20_39(23, C, D, E, A, B);
T_20_39(24, B, C, D, E, A);
T_20_39(25, A, B, C, D, E);
T_20_39(26, E, A, B, C, D);
T_20_39(27, D, E, A, B, C);
T_20_39(28, C, D, E, A, B);
T_20_39(29, B, C, D, E, A);
T_20_39(30, A, B, C, D, E);
T_20_39(31, E, A, B, C, D);
T_20_39(32, D, E, A, B, C);
T_20_39(33, C, D, E, A, B);
T_20_39(34, B, C, D, E, A);
T_20_39(35, A, B, C, D, E);
T_20_39(36, E, A, B, C, D);
T_20_39(37, D, E, A, B, C);
T_20_39(38, C, D, E, A, B);
T_20_39(39, B, C, D, E, A);
/* Round 3 */
T_40_59(40, A, B, C, D, E);
T_40_59(41, E, A, B, C, D);
T_40_59(42, D, E, A, B, C);
T_40_59(43, C, D, E, A, B);
T_40_59(44, B, C, D, E, A);
T_40_59(45, A, B, C, D, E);
T_40_59(46, E, A, B, C, D);
T_40_59(47, D, E, A, B, C);
T_40_59(48, C, D, E, A, B);
T_40_59(49, B, C, D, E, A);
T_40_59(50, A, B, C, D, E);
T_40_59(51, E, A, B, C, D);
T_40_59(52, D, E, A, B, C);
T_40_59(53, C, D, E, A, B);
T_40_59(54, B, C, D, E, A);
T_40_59(55, A, B, C, D, E);
T_40_59(56, E, A, B, C, D);
T_40_59(57, D, E, A, B, C);
T_40_59(58, C, D, E, A, B);
T_40_59(59, B, C, D, E, A);
/* Round 4 */
T_60_79(60, A, B, C, D, E);
T_60_79(61, E, A, B, C, D);
T_60_79(62, D, E, A, B, C);
T_60_79(63, C, D, E, A, B);
T_60_79(64, B, C, D, E, A);
T_60_79(65, A, B, C, D, E);
T_60_79(66, E, A, B, C, D);
T_60_79(67, D, E, A, B, C);
T_60_79(68, C, D, E, A, B);
T_60_79(69, B, C, D, E, A);
T_60_79(70, A, B, C, D, E);
T_60_79(71, E, A, B, C, D);
T_60_79(72, D, E, A, B, C);
T_60_79(73, C, D, E, A, B);
T_60_79(74, B, C, D, E, A);
T_60_79(75, A, B, C, D, E);
T_60_79(76, E, A, B, C, D);
T_60_79(77, D, E, A, B, C);
T_60_79(78, C, D, E, A, B);
T_60_79(79, B, C, D, E, A);
ctx->H[0] += A;
ctx->H[1] += B;
ctx->H[2] += C;
ctx->H[3] += D;
ctx->H[4] += E;
}
void blk_SHA1_Init(blk_SHA_CTX *ctx)
{
ctx->size = 0;
/* Initialize H with the magic constants (see FIPS180 for constants) */
ctx->H[0] = 0x67452301;
ctx->H[1] = 0xefcdab89;
ctx->H[2] = 0x98badcfe;
ctx->H[3] = 0x10325476;
ctx->H[4] = 0xc3d2e1f0;
}
void blk_SHA1_Update(blk_SHA_CTX *ctx, const void *data, size_t len)
{
unsigned int lenW = ctx->size & 63;
ctx->size += len;
/* Read the data into W and process blocks as they get full */
if (lenW) {
unsigned int left = 64 - lenW;
if (len < left)
left = len;
memcpy(lenW + (char *)ctx->W, data, left);
lenW = (lenW + left) & 63;
len -= left;
data = ((const char *)data + left);
if (lenW)
return;
blk_SHA1_Block(ctx, ctx->W);
}
while (len >= 64) {
blk_SHA1_Block(ctx, data);
data = ((const char *)data + 64);
len -= 64;
}
if (len)
memcpy(ctx->W, data, len);
}
void blk_SHA1_Final(unsigned char hashout[20], blk_SHA_CTX *ctx)
{
static const unsigned char pad[64] = { 0x80 };
unsigned int padlen[2];
int i;
/* Pad with a binary 1 (ie 0x80), then zeroes, then length */
padlen[0] = htonl((uint32_t)(ctx->size >> 29));
padlen[1] = htonl((uint32_t)(ctx->size << 3));
i = ctx->size & 63;
blk_SHA1_Update(ctx, pad, 1 + (63 & (55 - i)));
blk_SHA1_Update(ctx, padlen, 8);
/* Output hash */
for (i = 0; i < 5; i++)
put_be32(hashout + i * 4, ctx->H[i]);
}
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