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 regi开发者_开发知识库sters 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?

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For x86-64, the ABI (PDF) encourages the absence of a frame pointer. The rationale is more or less "we have DWARF now, so it's not necessary for debugging or exception unwinding; if we make it optional from day one, then no software will come to depend on its existence."

x86-64 does have more registers than x86-32, but it still doesn't have enough. Freeing up more general-purpose registers is always a Good Thing from a compiler's point of view. The operations that require a stack crawl are slower, yes, but they are rare events, so it's a good tradeoff for shaving a few cycles off every subroutine call plus fewer stack spills.