build: fix clang -Wshorten-64-to-32 warnings by csjones · Pull Request #1801 · bitcoin-core/secp256k1

csjones · 2026-01-15T00:58:33Z

Addressing most of the remaining build warnings from issue thread #1617.

I used master branch @ 4721e07 with command:

cmake -B build \
  -DCMAKE_C_COMPILER=clang \
  -DSECP256K1_ENABLE_MODULE_ECDH=ON \
  -DSECP256K1_ENABLE_MODULE_RECOVERY=ON \
  -DSECP256K1_ENABLE_MODULE_EXTRAKEYS=ON \
  -DSECP256K1_ENABLE_MODULE_SCHNORRSIG=ON \
  -DSECP256K1_ENABLE_MODULE_MUSIG=ON \
  -DSECP256K1_ENABLE_MODULE_ELLSWIFT=ON \
  -DSECP256K1_BUILD_TESTS=OFF \
  -DSECP256K1_BUILD_EXHAUSTIVE_TESTS=OFF \
  -DSECP256K1_BUILD_CTIME_TESTS=OFF \
  -DSECP256K1_BUILD_BENCHMARK=OFF \
  -DSECP256K1_BUILD_EXAMPLES=OFF \
  -DCMAKE_C_FLAGS="-Wall -Wextra -Wshorten-64-to-32 -Werror=shorten-64-to-32"
  
cmake --build build --verbose

which produced these 14 errors:

/Users/csjones/Developer/secp256k1/src/modinv64_impl.h:279:39: error: implicit conversion loses integer precision: 'uint64_t' (aka 'unsigned long long') to 'uint32_t' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
  279 |             w = (f * g * (f * f - 2)) & m;
      |               ~ ~~~~~~~~~~~~~~~~~~~~~~^~~
/Users/csjones/Developer/secp256k1/src/modinv64_impl.h:289:19: error: implicit conversion loses integer precision: 'uint64_t' (aka 'unsigned long long') to 'uint32_t' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
  289 |             w = f + (((f + 1) & 4) << 1);
      |               ~ ~~^~~~~~~~~~~~~~~~~~~~~~
/Users/csjones/Developer/secp256k1/src/modinv64_impl.h:290:26: error: implicit conversion loses integer precision: 'uint64_t' (aka 'unsigned long long') to 'uint32_t' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
  290 |             w = (-w * g) & m;
      |               ~ ~~~~~~~~~^~~
/Users/csjones/Developer/secp256k1/src/modinv64_impl.h:370:39: error: implicit conversion loses integer precision: 'uint64_t' (aka 'unsigned long long') to 'uint32_t' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
  370 |             w = (f * g * (f * f - 2)) & m;
      |               ~ ~~~~~~~~~~~~~~~~~~~~~~^~~
/Users/csjones/Developer/secp256k1/src/modinv64_impl.h:380:19: error: implicit conversion loses integer precision: 'uint64_t' (aka 'unsigned long long') to 'uint32_t' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
  380 |             w = f + (((f + 1) & 4) << 1);
      |               ~ ~~^~~~~~~~~~~~~~~~~~~~~~
/Users/csjones/Developer/secp256k1/src/modinv64_impl.h:381:26: error: implicit conversion loses integer precision: 'uint64_t' (aka 'unsigned long long') to 'uint32_t' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
  381 |             w = (-w * g) & m;
      |               ~ ~~~~~~~~~^~~
In file included from /Users/csjones/Developer/secp256k1/src/secp256k1.c:28:
In file included from /Users/csjones/Developer/secp256k1/src/scalar_impl.h:20:
/Users/csjones/Developer/secp256k1/src/scalar_4x64_impl.h:112:42: error: implicit conversion loses integer precision: 'uint64_t' (aka 'unsigned long long') to 'int' [-Werror,-Wshorten-64-to-32]
  112 |     overflow = secp256k1_u128_to_u64(&t) + secp256k1_scalar_check_overflow(r);
      |              ~ ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/csjones/Developer/secp256k1/src/scalar_4x64_impl.h:637:10: error: implicit conversion loses integer precision: 'uint64_t' (aka 'unsigned long long') to 'uint32_t' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
  637 |     m6 = c0;
      |        ~ ^~
/Users/csjones/Developer/secp256k1/src/scalar_4x64_impl.h:657:13: error: implicit conversion loses integer precision: 'uint64_t' (aka 'unsigned long long') to 'uint32_t' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
  657 |     p4 = c0 + m6;
      |        ~ ~~~^~~~
/Users/csjones/Developer/secp256k1/src/scalar_4x64_impl.h:677:34: error: implicit conversion loses integer precision: 'uint64_t' (aka 'unsigned long long') to 'unsigned int' [-Werror,-Wshorten-64-to-32]
  677 |     secp256k1_scalar_reduce(r, c + secp256k1_scalar_check_overflow(r));
      |     ~~~~~~~~~~~~~~~~~~~~~~~    ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /Users/csjones/Developer/secp256k1/src/secp256k1.c:30:
/Users/csjones/Developer/secp256k1/src/ecmult_impl.h:536:21: error: implicit conversion loses integer precision: 'size_t' (aka 'unsigned long') to 'int' [-Werror,-Wshorten-64-to-32]
  536 |     for (i = n_wnaf - 1; i >= 0; i--) {
      |            ~ ~~~~~~~^~~
/Users/csjones/Developer/secp256k1/src/ecmult_impl.h:580:52: error: implicit conversion loses integer precision: 'long' to 'int' [-Werror,-Wshorten-64-to-32]
  580 |         for(j = ECMULT_TABLE_SIZE(bucket_window+2) - 1; j > 0; j--) {
      |               ~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
In file included from /Users/csjones/Developer/secp256k1/src/secp256k1.c:32:
In file included from /Users/csjones/Developer/secp256k1/src/ecmult_gen_impl.h:14:
/Users/csjones/Developer/secp256k1/src/hash_impl.h:151:52: error: implicit conversion loses integer precision: 'uint64_t' (aka 'unsigned long long') to 'uint32_t' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
  151 |     secp256k1_write_be32(&sizedesc[0], hash->bytes >> 29);
      |     ~~~~~~~~~~~~~~~~~~~~               ~~~~~~~~~~~~^~~~~
/Users/csjones/Developer/secp256k1/src/hash_impl.h:152:52: error: implicit conversion loses integer precision: 'uint64_t' (aka 'unsigned long long') to 'uint32_t' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
  152 |     secp256k1_write_be32(&sizedesc[4], hash->bytes << 3);
      |     ~~~~~~~~~~~~~~~~~~~~               ~~~~~~~~~~~~^~~~
14 errors generated.

I applied most of the suggested fixes from #1617 (comment) and used an explicit cast of (uint32_t) in scalar_4x64_impl.h for the two warnings not mentioned in the original issue thread. The warnings in ecmult_impl.h were skipped.

Only two warnings remain after applying the changes from this PR.

/Users/csjones/Developer/secp256k1/src/ecmult_impl.h:536:21: error: implicit conversion loses integer precision: 'size_t' (aka 'unsigned long') to 'int' [-Werror,-Wshorten-64-to-32]
  536 |     for (i = n_wnaf - 1; i >= 0; i--) {
      |            ~ ~~~~~~~^~~
/Users/csjones/Developer/secp256k1/src/ecmult_impl.h:580:52: error: implicit conversion loses integer precision: 'long' to 'int' [-Werror,-Wshorten-64-to-32]
  580 |         for(j = ECMULT_TABLE_SIZE(bucket_window+2) - 1; j > 0; j--) {
      |               ~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
2 errors generated.

EDIT: ~~Investigating failing CI~~ Fixed.

…lementation

…ion' warnings in scalar_4x64_impl.h

real-or-random · 2026-01-21T16:12:02Z

Can you squash the last two commits?

…ion' warnings in hash_impl.h fix: correct cast placement in hash_impl.h to preserve full 64-bit shift operations

csjones · 2026-01-21T16:55:07Z

@real-or-random just pushed the squashed commit.

real-or-random

cc @sipa for the w change... Is this correct even? Even if it's correct, do we want to keep the smaller uint32_t?

src/scalar_4x64_impl.h

real-or-random · 2026-01-21T19:32:34Z

src/scalar_4x64_impl.h

+    p4 = (uint32_t)c0 + m6;
    VERIFY_CHECK(p4 <= 2);


Suggested change

p4 = (uint32_t)c0 + m6;

VERIFY_CHECK(p4 <= 2);

VERIFY_CHECK(c0 <= 1 && m6 <= 1);

p4 = (uint32_t)c0 + m6;

Change made

I ended up removing m6 <= 1 because it's already verified at line 637. The CI failed with c0 <= 1 and after running the ci script locally a few times, I changed it to c0 <= 2, which is consistent with the original p4 <= 2 check (since p4 = c0 + m6 and m6 <= 1). I also restored the p4 <= 2 check.

Hm, I see. I have no idea how the algorithm works. Still, this is a bit surprising to me because that means m6 == 0 always (but then it probably wouldn't be there in the first place) or m6 == 0 iff c0 == 2, and I don't see why the latter should hold.

Perhaps @sipa can provide some insights..

src/scalar_4x64_impl.h

…asts in scalar_4x64_impl.h

csjones added 2 commits January 14, 2026 14:42

fix: change w variable type from uint32_t to uint64_t in modinv64 imp…

66452d4

…lementation

fix: use explicit casts for 'implicit conversion loses integer precis…

3ddc5be

…ion' warnings in scalar_4x64_impl.h

csjones mentioned this pull request Jan 15, 2026

"implicit conversion loses integer precision" warnings #1617

Open

real-or-random added the tweak/refactor label Jan 21, 2026

fix: use explicit casts for 'implicit conversion loses integer precis…

4a84fb7

…ion' warnings in hash_impl.h fix: correct cast placement in hash_impl.h to preserve full 64-bit shift operations

csjones force-pushed the CLANG-WARN-WSHORTEN-64-TO-32 branch from 98749c3 to 4a84fb7 Compare January 21, 2026 16:52

real-or-random reviewed Jan 21, 2026

View reviewed changes

refactor: preserve unsigned arithmetic and add VERIFY_CHECKs before c…

da9db68

…asts in scalar_4x64_impl.h

csjones force-pushed the CLANG-WARN-WSHORTEN-64-TO-32 branch from 9e3dc93 to da9db68 Compare January 22, 2026 06:26

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Comments

build: fix clang -Wshorten-64-to-32 warnings#1801

build: fix clang -Wshorten-64-to-32 warnings#1801
csjones wants to merge 4 commits intobitcoin-core:masterfrom
21-DOT-DEV:CLANG-WARN-WSHORTEN-64-TO-32

csjones commented Jan 15, 2026 •

edited

Loading

Uh oh!

real-or-random commented Jan 21, 2026

Uh oh!

csjones commented Jan 21, 2026

Uh oh!

real-or-random left a comment

Uh oh!

Uh oh!

real-or-random Jan 21, 2026

Uh oh!

csjones Jan 22, 2026

Uh oh!

csjones Jan 22, 2026

Uh oh!

real-or-random Jan 22, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

Comments

Conversation

csjones commented Jan 15, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

real-or-random commented Jan 21, 2026

Uh oh!

csjones commented Jan 21, 2026

Uh oh!

real-or-random left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

real-or-random Jan 21, 2026

Choose a reason for hiding this comment

Uh oh!

csjones Jan 22, 2026

Choose a reason for hiding this comment

Uh oh!

csjones Jan 22, 2026

Choose a reason for hiding this comment

Uh oh!

real-or-random Jan 22, 2026

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

csjones commented Jan 15, 2026 •

edited

Loading