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copied!<blockquote> <p>I'd like to write a template function</p> <pre><code>template <typename T> void f( T v ); </code></pre> <p>such that <code>v</code> will be passed by value if it's small enough, otherwise by reference-to-const.</p> </blockquote> <p><strong>The compiler can be smart enough to do the right thing without any template magic,</strong> especially if the function <code>f</code> can be inlined. I would always implement <code>f</code> as </p> <pre><code>template <typename T> void f(const T& v); </code></pre> <p>and trust the compiler to turn that into a copy if the copy is cheaper.</p> <p>Here is an example:</p> <pre><code>extern volatile int k; extern volatile int m; static void f(const int& j) noexcept { // or f(const int j) for (int i=0; i<j; ++i) { m = i; } } void g() noexcept { int j = k; f(j); } </code></pre> <p>I ran <code>clang++ -O3 -std=c++11 -S -emit-llvm filename.cpp</code> and the generated assembly (as far as I can tell) is the same.</p> <p>Pass by reference:</p> <pre><code>@k = external global i32 @m = external global i32 ; Function Attrs: nounwind uwtable define void @_Z1gv() #0 { entry: %0 = load volatile i32* @k, align 4, !tbaa !0 %cmp3.i = icmp sgt i32 %0, 0 br i1 %cmp3.i, label %for.body.i, label %_ZL1fRKi.exit for.body.i: ; preds = %entry, %for.body.i %i.04.i = phi i32 [ %inc.i, %for.body.i ], [ 0, %entry ] store volatile i32 %i.04.i, i32* @m, align 4, !tbaa !0 %inc.i = add nsw i32 %i.04.i, 1 %exitcond = icmp eq i32 %inc.i, %0 br i1 %exitcond, label %_ZL1fRKi.exit, label %for.body.i _ZL1fRKi.exit: ; preds = %for.body.i, %entry ret void } </code></pre> <p>Pass by value:</p> <pre><code>@k = external global i32 @m = external global i32 ; Function Attrs: nounwind uwtable define void @_Z1gv() #0 { entry: %0 = load volatile i32* @k, align 4, !tbaa !0 %cmp3.i = icmp sgt i32 %0, 0 br i1 %cmp3.i, label %for.body.i, label %_ZL1fi.exit for.body.i: ; preds = %entry, %for.body.i %i.04.i = phi i32 [ %inc.i, %for.body.i ], [ 0, %entry ] store volatile i32 %i.04.i, i32* @m, align 4, !tbaa !0 %inc.i = add nsw i32 %i.04.i, 1 %exitcond.i = icmp eq i32 %inc.i, %0 br i1 %exitcond.i, label %_ZL1fi.exit, label %for.body.i _ZL1fi.exit: ; preds = %for.body.i, %entry ret void } </code></pre> <p>If <code>f</code> is not inlined then the assembly is different though.</p>

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