GCC builtins in LDC - compiler interoperability

Cecil Ward cecil at cecilward.com
Wed Jul 15 23:31:39 UTC 2020

On Wednesday, 15 July 2020 at 08:20:48 UTC, kinke wrote:
> On Tuesday, 14 July 2020 at 23:37:13 UTC, Cecil Ward wrote:
>> I currently have a fair amount of code that will only run 
>> under GDC because of reliance on GDC-specific library 
>> routines. I’d like to ask whether I can find or implement 
>> equivalents in LDC2 to the GDC routines
>>         __builtin_expect()
>> and
>>         __builtin_unreachable() ?
> As Ilya already posted in another thread of yours, 
> __builtin_expect is ldc.intrinsics.llvm_expect. Many other gcc 
> builtins are available in ldc.gccbuiltins_<arch>. I don't think 
> we expose `unreachable` though.

Oh, shame. There’s no __builtin_unreachable in ldc? I wonder if I 
could work around this with LDC then.

I’m using unreachable to declare the truth of a condition in 
release build so that the compiler can generate better code. The 
trick I use with GCC is to mark the body of some if-statement as 
unreachable. Then the compiler can remove a load of later if-test 
conditions or  remove if/else statements’ basic blocks.

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