[llvm-commits] patch: target option to disable tail calls

Eli Friedman eli.friedman at gmail.com
Wed Jan 18 16:13:48 PST 2012

On Wed, Jan 18, 2012 at 4:02 PM, Nick Lewycky <nlewycky at google.com> wrote:
> On 18 January 2012 15:25, Eli Friedman <eli.friedman at gmail.com> wrote:
>> On Wed, Jan 18, 2012 at 3:10 PM, Nick Lewycky <nlewycky at google.com> wrote:
>> > The attached patch adds a new "-no-tail-calls" flag to llc, and
>> > implements
>> > it in the x86 backend only. The intent is to support gcc's
>> > -fno-optimize-sibling-calls in clang, which as far as I can tell just
>> > disables tail calls period. The goal is to help ASAN produce correct
>> > stack
>> > traces, which requires disabling all tail calls.
>> >
>> > As an aside, there is a good reason to do this in the backend and not at
>> > the
>> > IR-level. The "tail" marker is used to indicate things that are useful
>> > to
>> > the optimizers themselves; for example alias analysis interprets the
>> > presence of a tail marker as proof that the callee can't access any
>> > alloca's
>> > in the caller, whether the pointer has been captured or not.
>> You're right that expressing this constraint with the tail marker
>> doesn't make sense; that said, why not a function attribute?
> Hunh? Why would you want to set this per-function? This is similar to
> choosing (codegen) optimization levels or an ABIs, or other similar codegen
> issues.
> For my use-case, I want all functions to be built this way.

Another thing to think about is "does it make sense for LTO to mix two
modules with different specifications for this option".  But from what
you're saying, it sounds like that doesn't really make sense, so it's


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