6fe4e033f0
Credits: this change is based on analysis and a proof of concept by gerbens@google.com. Before, the compiler loses track of end as 'this' and other references possibly escape beyond the compiler's scope. This can be see in the generated assembly: 16.28 │200c80: mov %r15d,(%rax) 60.87 │200c83: add $0x4,%rax │200c87: mov %rax,-0x38(%rbp) 0.03 │200c8b: → jmpq 200d4e ... ... 1.69 │200d4e: cmp %r15d,%r12d │200d51: → je 200c40 16.34 │200d57: inc %r15d 0.05 │200d5a: mov -0x38(%rbp),%rax 3.27 │200d5e: mov -0x30(%rbp),%r13 1.47 │200d62: cmp %r13,%rax │200d65: → jne 200c80 We fix this by always explicitly storing the loaded local and pointer back at the end of push back. This generates some slight source 'noise', but creates nice and compact fast path code, i.e.: 32.64 │200760: mov %r14d,(%r12) 9.97 │200764: add $0x4,%r12 6.97 │200768: mov %r12,-0x38(%rbp) 32.17 │20076c: add $0x1,%r14d 2.36 │200770: cmp %r14d,%ebx │200773: → je 200730 8.98 │200775: mov -0x30(%rbp),%r13 6.75 │200779: cmp %r13,%r12 │20077c: → jne 200760 Now there is a single store for the push_back value (as before), and a single store for the end without a reload (dependency). For fully local vectors, (i.e., not referenced elsewhere), the capacity load and store inside the loop could also be removed, but this requires more substantial refactoring inside vector. Differential Revision: https://reviews.llvm.org/D80588
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