Re: [PATCH] Fix PR87691: transparent_union attribute does not work with MODE_PARTIAL_INT

[Marek Polacek <polacek at redhat dot com>]

On Tue, Oct 23, 2018 at 08:49:26PM +0100, Jozef Lawrynowicz wrote:
> msp430-elf uses the partial int type __int20 for pointers in the large memory
> model. __int20 has PSImode, with bitsize of 20.
> 
> A few DejaGNU tests fail when built with -mlarge for msp430-elf, when
> transparent unions are used containing pointers.
> These are:
> - gcc.c-torture/compile/pr34885.c
> - gcc.dg/transparent-union-{1,2,3,4,5}.c
> 
> The issue is that the union is considered to have size of 32 bits (the
> in-memory size of __int20), so unless mode_for_size as called by
> compute_record_mode (both in stor-layout.c) is explicitly told to look for a
> mode of class MODE_PARTIAL_INT, then a size of 32 will always return MODE_INT.
> In this case, the union will have TYPE_MODE of SImode, but its field is
> PSImode, so transparent_union has no effect.
> 
> The attached patch fixes the issue by allowing the TYPE_MODE of a union to be
> set to the DECL_MODE of the widest field, if the mode is of class
> MODE_PARTIAL_INT and the union would be passed by reference.
> 
> Some target ABIs mandate that unions be passed in integer registers, so to
> avoid any potential ABI violations, the mode of the union is only changed if
> it would be passed by reference.
> 
> Successfully bootstrapped and regstested trunk for x86_64-pc-linux-gnu, and
> msp430-elf with -mlarge. For msp430-elf with -mlarge, the above DejaGNU tests
> are also fixed.
> 
> Ok for trunk?
> 

> From cc1ccfcc0d8adf7b0e1ca95a47a8a8e7e12fc99c Mon Sep 17 00:00:00 2001
> From: Jozef Lawrynowicz <jozef.l@mittosystems.com>
> Date: Mon, 22 Oct 2018 21:02:10 +0100
> Subject: [PATCH] Allow union TYPE_MODE to be set to the mode of the widest
>  element if the union would be passed by reference
> 
> 2018-10-23  Jozef Lawrynowicz  <jozef.l@mittosystems.com>
> 
> 	PR c/87691
> 	* gcc/stor-layout.c (compute_record_mode): Set TYPE_MODE of UNION_TYPE
> 	to the mode of the widest field iff the widest field has mode class
> 	MODE_INT, or MODE_PARTIAL_INT and the union would be passed by
> 	reference.
> 	* gcc/testsuite/gcc.target/msp430/pr87691.c: New test.

I'll just point out that you should drop the gcc/ and gcc/testsuite/ prefixes;
the first entry will go to gcc/ChangeLog while the pr87691.c one to
gcc/testsuite/ChangeLog.

Marek