'Why does GCC drop the frame pointer on 64-bit?

What's the rationale behind dropping the frame pointer on 64-bit architectures by default? I'm well aware that it can be enabled but why does GCC disable it in the first place while having it enabled for 32-bit? After all, 64-bit has more registers than 32-bit CPUs.

Edit:

Looks like the frame pointer will be also dropped for x86 when using a more recent GCC version. From the manual:

Starting with GCC version 4.6, the default setting (when not optimizing for size) for 32-bit Linux x86 and 32-bit Darwin x86 targets has been changed to -fomit-frame-pointer. The default can be reverted to -fno-omit-frame-pointer by configuring GCC with the --enable-frame-pointer configure option.

But why?

gccassemblyx8664-bitstack-frame


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