RFR: 8261812: C2 compilation fails with assert(!had_error) failed: bad dominance

[Vladimir Kozlov]

On Mon, 1 Mar 2021 07:51:40 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> In the testByte() test case, byteField is assigned int values that
> don't fit in a byte.
> 
> obj.byteField = (byte)(1 << i);
> 
> At parse time, c2 expands this to (in pseudo code):
> 
> int v = (1 << i);
> obj.byteField = (v << 24) >> 24;
> 
> next:
> 
> StoreBNode::Ideal() runs and StoreNode::Ideal_sign_extended_input()
> causes this to become:
> 
> obj.byteField = (1 << i);
> 
> i is then constant folded to 9:
> 
> obj.byteField = 512;
> 
> obj doesn't escape and is scalarized. The value of the byteField needs
> to be recorded in the debug info. Because there are 2 such a store, a
> Phi is created:
> 
> (Phi 512 1024)
> 
> The Phi is created with type TypeInt::BYTE. Because that doesn't
> overlap with the input values, the Phi is transformed to top. The test
> case then triggers a deoptimization and a crash occurs in the
> deoptimization code.
> 
> The failure of the bug report is not a crash during deoptimization but
> a c2 crash. The root cause is the same but in that case a chain of Phi
> is involved and when one becomes top, it causes an uncommon trap to
> have an input that doesn't dominate the uncommon trap.
> 
> I noticed LoadBNode::Ideal() re-inserts the 2 shifts when it returns a
> value from a Store. I propose the same fix here in the allocation
> elimination code.
> 
> Same problem applies to short and char. They are covered by the fix. I
> also included boolean even though I don't see how to trigger the
> failure with it.

Otherwise looks good.

src/hotspot/share/opto/macro.cpp line 453:

> 451:         BarrierSetC2* bs = BarrierSet::barrier_set()->barrier_set_c2();
> 452:         n = bs->step_over_gc_barrier(n);
> 453:         if (ft == T_SHORT || ft == T_BYTE || ft == T_CHAR || ft == T_BOOLEAN) {

Use `is_subword_type(bt)` defined in `globalDefinitions.hpp`

-------------

Changes requested by kvn (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2774