Re: [expand] Fix for PR rtl-optimization/79121 incorrect expansion of extend plus left shift

[Jeff Law <law at redhat dot com>]

On 01/18/2017 11:08 AM, Richard Earnshaw (lists) wrote:
> PR 79121 is a silent wrong code regression where, when generating a
> shift from an extended value moving from one to two machine registers,
> the type of the right shift is for the most significant word should be
> determined by the signedness of the inner type, not the signedness of
> the result type.
>
> gcc:
> 	PR rtl-optimization/79121
> 	* expr.c (expand_expr_real_2, case LSHIFT_EXPR): Look at the signedness
> 	of the inner type when shifting an extended value.
>
> testsuite:
> 	* gcc.c-torture/execute/pr79121.c: New test.
>
> Bootstrapped on x86_64 and cross-tested on ARM.
I had to refamiliarize myself with this code and nearly got the analysis 
wrong (again).

Due to the copying of the low word into the high word we have to select 
the type of shift based on the type of the object that was the source of 
the NOP conversion.  The code currently makes that determination based 
on the signedness of the shift, which is wrong.


OK for the trunk.

jeff