Avoids a potentially shortening size_t to int cast in strauss_wnaf_

[real-or-random]

Fixes #834.

[sipa]

I confirm this fixes the

In file included from src/secp256k1.c:16:
src/ecmult_impl.h: In function ‘secp256k1_ecmult.constprop’:
src/ecmult_impl.h:496:48: warning: array subscript [1, 268435456] is outside array bounds of ‘struct secp256k1_strauss_point_state[1]’ [-Warray-bounds]
  496 |             secp256k1_gej tmp = a[state->ps[np].input_pos];
      |                                   ~~~~~~~~~~~~~^~~~~~~~~~
src/ecmult_impl.h:565:42: note: while referencing ‘ps’
  565 |     struct secp256k1_strauss_point_state ps[1];
      |                                          ^~
src/ecmult_impl.h:502:139: warning: array subscript [1, 268435456] is outside array bounds of ‘struct secp256k1_strauss_point_state[1]’ [-Warray-bounds]
  502 |             secp256k1_fe_mul(state->zr + np * ECMULT_TABLE_SIZE(WINDOW_A), state->zr + np * ECMULT_TABLE_SIZE(WINDOW_A), &(a[state->ps[np].input_pos].z));
      |                                                                                                                              ~~~~~~~~~~~~~^~~~~~~~~~
src/ecmult_impl.h:565:42: note: while referencing ‘ps’
  565 |     struct secp256k1_strauss_point_state ps[1];

warning.

[hebasto]

Tested f1c9131 on Linux Mint 20 (x86_64):

$ gcc --version
gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ make > /dev/null
# no warnings :)

[gmaxwell]

This looks reasonable to me.

[real-or-random]

Ok, I didn't expect that this will also fix the warning! Updated the commit message accordingly.

[sipa]

Ok, I didn't expect that this will also fix the warning! Updated the commit message accordingly.

Bogus compiler warning acts bogusly.

[sipa]

ACK 8893f42. np and no shouldn't ever take on negative values.

[jonasnick]

ACK 8893f42

Also kicked travis since it hadn't started the job.

[elichai]

ACK 8893f42
Cool that it fixed the warning hehe

[hebasto]

Is it planned to pull this change into the Bitcoin Core repo before 0.21.0 branching off?

[gmaxwell]

@hebasto You're asking in the wrong place, other than sipa no one else here regularly contributes to bitcoin-core. :)