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

[Björn Steinbrink]

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