[gcc(refs/users/aoliva/heads/testme)] Reject unaligned subregs when strict alignment is required

[Alexandre Oliva <aoliva@gcc.gnu.org>]

https://gcc.gnu.org/g:3d8a853f35a2a5e259d84c26580224d4d1de10e8

commit 3d8a853f35a2a5e259d84c26580224d4d1de10e8
Author: Alexandre Oliva <oliva@adacore.com>
Date:   Wed May 4 23:41:39 2022 -0300

    Reject unaligned subregs when strict alignment is required
    
    The testcase for pr100106, compiled with optimization for 32-bit
    powerpc -mcpu=604 with -mstrict-align expands the initialization of a
    union from a float _Complex value into a load from an SCmode
    constant pool entry, aligned to 4 bytes, into a DImode pseudo,
    requiring 8-byte alignment.
    
    The patch that introduced the testcase modified simplify_subreg to
    avoid changing the MEM to outermode, but simplify_gen_subreg still
    creates a SUBREG or a MEM that would require stricter alignment than
    MEM's, and lra_constraints appears to get confused by that, repeatedly
    creating unsatisfiable reloads for the SUBREG until it exceeds the
    insn count.
    
    Avoiding the unaligned SUBREG, expand splits the DImode dest into
    SUBREGs and loads each SImode word of the constant pool with the
    proper alignment.
    
    
    for  gcc/ChangeLog
    
            PR target/100106
            * emit-rtl.c (validate_subreg): Reject a SUBREG of a MEM that
            requires stricter alignment than MEM's.
    
    for  gcc/testsuite/ChangeLog
    
            PR target/100106
            * gcc.target/powerpc/pr100106-sa.c: New.
    
    TN: V502-036

Diff:
---
 gcc/emit-rtl.cc                                | 3 +++
 gcc/testsuite/gcc.target/powerpc/pr100106-sa.c | 4 ++++
 2 files changed, 7 insertions(+)

diff --git a/gcc/emit-rtl.cc b/gcc/emit-rtl.cc
index 1e02ae254d0..642e47eada0 100644
--- a/gcc/emit-rtl.cc
+++ b/gcc/emit-rtl.cc
@@ -982,6 +982,9 @@ validate_subreg (machine_mode omode, machine_mode imode,
 
       return subreg_offset_representable_p (regno, imode, offset, omode);
     }
+  else if (reg && MEM_P (reg)
+	   && STRICT_ALIGNMENT && MEM_ALIGN (reg) < GET_MODE_ALIGNMENT (omode))
+    return false;
 
   /* The outer size must be ordered wrt the register size, otherwise
      we wouldn't know at compile time how many registers the outer
diff --git a/gcc/testsuite/gcc.target/powerpc/pr100106-sa.c b/gcc/testsuite/gcc.target/powerpc/pr100106-sa.c
new file mode 100644
index 00000000000..1cd6b595a5b
--- /dev/null
+++ b/gcc/testsuite/gcc.target/powerpc/pr100106-sa.c
@@ -0,0 +1,4 @@
+/* { dg-do compile } */
+/* { dg-options "-mcpu=604 -O -mstrict-align" } */
+
+#include "../../gcc.c-torture/compile/pr100106.c"