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Merge bitcoin/bitcoin#25709: script: actually trigger the optimization in BuildScript

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00897d0677 script: actually trigger the optimization in BuildScript (Antoine Poinsot)

Pull request description:

  The counter is an optimization over calling `ret.empty()`. It was
  suggested that the compiler would realize `cnt` is only `0` on the first
  iteration, and not actually emit the check and conditional.

  This optimization was actually not triggered at all, since we
  incremented `cnt` at the beginning of the first iteration. Fix it by
  incrementing at the end instead.

  This was reported by Github user "Janus".

  Fixes #25682. Note this does *not* change semantics. It only allows the optimization of moving instead of copying on first `CScript` element to actually be reachable.

ACKs for top commit:
  sipa:
    utACK 00897d0677
  MarcoFalke:
    review ACK 00897d0677

Tree-SHA512: b575bd444b0cd2fe754ec5f3e2f3f53d2696d5dcebedcace1e38be372c82365e75938dfe185429ed5a83efe1a395e204bfb33efe56c10defc5811eaee50580e3
This commit is contained in:
MacroFake 2022-07-30 17:48:31 +02:00
commit 5215c80edc
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1 changed files with 1 additions and 1 deletions
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src/script/script.h
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@ -588,7 +588,6 @@ CScript BuildScript(Ts&&... inputs)
int cnt{0}; int cnt{0};
([&ret, &cnt] (Ts&& input) { ([&ret, &cnt] (Ts&& input) {
cnt++;
if constexpr (std::is_same_v<std::remove_cv_t<std::remove_reference_t<Ts>>, CScript>) { if constexpr (std::is_same_v<std::remove_cv_t<std::remove_reference_t<Ts>>, CScript>) {
// If it is a CScript, extend ret with it. Move or copy the first element instead. // If it is a CScript, extend ret with it. Move or copy the first element instead.
if (cnt == 0) { if (cnt == 0) {
@ -600,6 +599,7 @@ CScript BuildScript(Ts&&... inputs)
// Otherwise invoke CScript::operator<<. // Otherwise invoke CScript::operator<<.
ret << input; ret << input;
} }
cnt++;
} (std::forward<Ts>(inputs)), ...); } (std::forward<Ts>(inputs)), ...);
return ret; return ret;