> Am 13.01.2026 um 22:39 schrieb Andrew MacLeod <[email protected]>:
>
>
>> On 1/12/26 10:23, Andrew MacLeod wrote:
>>
>>> On 1/6/26 12:35, Andrew MacLeod wrote:
>>> VRP1 is allowed to remove a __builtin_unreachable () if it determines that
>>> the other edge in the branch dominates all uses of all exports from the
>>> block.
>>>
>>> ie
>>>
>>> <bb 3>:
>>> x_8 = 1 << i_7;
>>> if (x_8 <= 0) <<-- this is the branch in BB3
>>> goto <bb 4>; [INV]
>>> else
>>> goto <bb 5>; [INV]
>>>
>>> <bb 4> :
>>> __builtin_unreachable ();
>>>
>>> we can remove the branch and edge to bb 4 if we can prove that there are
>>> no uses of x_8 or i_7 in or dominated by the edge 3->5. We can then set
>>> the global ranges of i_7 and x_8 and move along.
>>>
>>> This PR demonstrates that this normally works, but with rangers ability to
>>> recompute values, we also have to look at the dependencies of all the
>>> exports.
>>>
>>> IN this case i_7 is defined:
>>> <bb 7> :
>>> # i_2 = PHI <i_7(5), n_4(D)(2), i_7(6)>
>>> i_7 = i_2 + -1;
>>> if (i_2 > 0)
>>> goto <bb 3>; [INV]
>>> else
>>> goto <bb 8>; [INV]
>>>
>>> so althoug the uses of i_7 Are dominated by 3->5, it does NOT dominate the
>>> use of i_2 in bb_7. When early removal changes the global value of i_7,
>>> ranger happily recomputes i_2 in the branch and decides that if i_7 is now
>>> [0, +INF], i_2 must always be > 0 and removes the branch.
>>>
>>> Which is clearly incorrect. This patch teaches the early removal code that
>>> it can only remove the unreachable call if x_8, i_7 and ALL the
>>> dependencies used to calculate either name are ALL dominated by the edge.
>>> This information is all trivially available in GORI, so its noly a minor
>>> tweak.
>>>
>>> Performance impact over a build of GCC is minimal. a 0.03% slowdown in VRP.
>>>
>>> In the PR I mentioned not removing it if any of the dependencies had more
>>> than a single use, but that turned out to be too limiting. This solution
>>> works much better.
>>>
>>> Bootstraps on x86_64-pc-linux-gnu with no regressions. OK?
>>>
>>> Andrew
>>>
>>>
>> Patch adjusted and applied to GCC-14. OK for branch or not?
>>
>> Andrew
>
> GCC15 patch was approved, is the same for GCC 14 as well?
Yes, sorry for not being clearer.
Richard
> Thanks
>
> Andrew
>
