Commit a71d00a4d504edfdb09cd169d29ca1bbc0b909c4 | zig

author	Andrew Kelley <andrew@ziglang.org> 2022-06-27 19:05:51-07:00
committer	Andrew Kelley <andrew@ziglang.org> 2022-06-27 19:11:55-07:00
log	a71d00a4d504edfdb09cd169d29ca1bbc0b909c4
tree	1b4f5a419ee46ccc26d605f00109f026d9a40c26
parent	0b8bd9b2b4f609fb4ae7d31da7e7a0fd4f5ad987

std.crypto.25519.field: avoid excessive inlining

This valid zig code produces reasonable LLVM IR, however, on the
wasm32-wasi target, when using the wasmtime runtime, the number of
locals of the `isSquare` function exceeds 50000, causing wasmtime
to refuse to execute the binary.

The `inline` keyword in Zig is intended to be used only where it is
semantically necessary; not as an optimization hint. Otherwise, this may
produce unwanted binary bloat for the -OReleaseSmall use case.

In the future, it is possible that we may end up with both `inline`
keyword, which operates as it does in status quo, and additionally
`callconv(.inline_hint)` which has no semantic impact, but may be
observed by optimization passes.

In this commit, I also cleaned up `isSquare` by eliminating an
unnecessary mutable variable, replacing it with several local constants.

Closes #11947.

1 files changed, 7 insertions(+), 8 deletions(-)

lib
- std
  - crypto
    25519
    field.zig

lib/std/crypto/25519/field.zig+7-8

 @@ -341,7 +341,7 @@ pub const Fe = struct {
     }
     /// Square a field element `n` times
-    inline fn sqn(a: Fe, comptime n: comptime_int) Fe {
+    fn sqn(a: Fe, n: usize) Fe {
         var i: usize = 0;
         var fe = a;
         while (i < n) : (i += 1) {
 @@ -390,13 +390,12 @@ pub const Fe = struct {
         const _11 = a.mul(a.sq());
         const _1111 = _11.mul(_11.sq().sq());
         const _11111111 = _1111.mul(_1111.sq().sq().sq().sq());
-        var t = _11111111.sqn(2).mul(_11);
+        const u = _11111111.sqn(2).mul(_11);
-        const u = t;
+        const t = u.sqn(10).mul(u).sqn(10).mul(u);
-        t = t.sqn(10).mul(u).sqn(10).mul(u);
+        const t2 = t.sqn(30).mul(t);
-        t = t.sqn(30).mul(t);
+        const t3 = t2.sqn(60).mul(t2);
-        t = t.sqn(60).mul(t);
+        const t4 = t3.sqn(120).mul(t3).sqn(10).mul(u).sqn(3).mul(_11).sq();
-        t = t.sqn(120).mul(t).sqn(10).mul(u).sqn(3).mul(_11).sq();
+        return @bitCast(bool, @truncate(u1, ~(t4.toBytes()[1] & 1)));
-        return @bitCast(bool, @truncate(u1, ~(t.toBytes()[1] & 1)));
     }
     fn uncheckedSqrt(x2: Fe) Fe {