[PATCH][GCC 14] PR tree-optimization/123300 - Early builtin_unreachable removal must examine dependencies.

[Andrew MacLeod]

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?

Thanks

Andrew