[LLVMdev] Why does "uwtable" prevent optimizing Invoke -> Call?

[Alexander Poddey]

Alexander Poddey wrote:
sorry this one had gone into the wrong thread!

> 
> Hi Björn,
> 
> :smiting my forehead: Sure! thats it, thanks a lot.
> 
> Alex
> 
>> Hi,
>> 
>> in r176827 the optimization that turns invokes with empty landing pads
>> into plain calls was disabled for invocations of function with the
>> "uwtable" attribute.
>> 
>> But given this code:
>> 
>>     struct S { ~S() {}; };
>> 
>>     void blackbox();
>>     __attribute__((__noinline__)) void inner() { blackbox(); }
>> 
>>     int foo() {
>>         S s;
>>         inner();
>>         return 0;
>>     }
>> 
>>     int bar() {
>>         inner();
>>         return 0;
>>     }
>> 
>> clang directly emits a call instruction in "bar", because there is
>> nothing to be cleaned up. But in "foo", it emits an invoke instruction
>> because of the S object. During optimization, the S object gets
>> optimized out and all that remains is the invoke instruction with an
>> empty landing pad, causing exception tables to be generated and in some
>> cases stopping other optimizations from being applied.
>> 
>> I don't see why code that has been completely optimized away should have
>> an effect on that function call. So AFAICT either I'm missing something
>> here, or clang should always emit invoke, disabling the optimization for
>> functions that have the uwtable attribute was wrong and the change
>> should be reverted.
>> 
>> FWIW, gcc does not generate exception tables for the above.
>> 
>> Björn
> 
> 
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