[gcc r14-2102] IBM zSystems: Assume symbols without explicit alignment to be ok

[gcc r14-2102] IBM zSystems: Assume symbols without explicit alignment to be ok

[Andreas Krebbel]

https://gcc.gnu.org/g:a29df49b7d8332f068926a45c950510ceeb79f4c

commit r14-2102-ga29df49b7d8332f068926a45c950510ceeb79f4c
Author: Andreas Krebbel <krebbel@linux.ibm.com>
Date:   Mon Jun 26 18:31:53 2023 +0200

    IBM zSystems: Assume symbols without explicit alignment to be ok
    
    A change we have committed back in 2015 relies on the backend
    requested ABI alignment to be applied to ALL symbols by the
    middle-end. However, this does not appear to be the case for external
    symbols. With this commit we assume all symbols without explicit
    alignment to be aligned according to the ABI. That's the behavior we
    had before.
    This fixes a performance regression caused by the 2015 patch. Since
    then the address of external char type symbols have been pushed to the
    literal pool, although it is safe to access them with larl (which
    requires symbols to reside at even addresses).
    
    gcc/
            * config/s390/s390.cc (s390_encode_section_info): Set
            SYMBOL_FLAG_SET_NOTALIGN2 only if the symbol has explicitely been
            misaligned.
    
    gcc/testsuite/
            * gcc.target/s390/larl-1.c: New test.

Diff:
---
 gcc/config/s390/s390.cc                |  6 ++++--
 gcc/testsuite/gcc.target/s390/larl-1.c | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/gcc/config/s390/s390.cc b/gcc/config/s390/s390.cc
index 9284477396d..d9f10542473 100644
--- a/gcc/config/s390/s390.cc
+++ b/gcc/config/s390/s390.cc
@@ -13706,8 +13706,10 @@ s390_encode_section_info (tree decl, rtx rtl, int first)
     {
       /* Store the alignment to be able to check if we can use
 	 a larl/load-relative instruction.  We only handle the cases
-	 that can go wrong (i.e. no FUNC_DECLs).  */
-      if (DECL_ALIGN (decl) == 0 || DECL_ALIGN (decl) % 16)
+	 that can go wrong (i.e. no FUNC_DECLs).
+	 All symbols without an explicit alignment are assumed to be 2
+	 byte aligned as mandated by our ABI.  */
+      if (DECL_USER_ALIGN (decl) && DECL_ALIGN (decl) % 16)
 	SYMBOL_FLAG_SET_NOTALIGN2 (XEXP (rtl, 0));
       else if (DECL_ALIGN (decl) % 32)
 	SYMBOL_FLAG_SET_NOTALIGN4 (XEXP (rtl, 0));
diff --git a/gcc/testsuite/gcc.target/s390/larl-1.c b/gcc/testsuite/gcc.target/s390/larl-1.c
new file mode 100644
index 00000000000..5ef2ef63f82
--- /dev/null
+++ b/gcc/testsuite/gcc.target/s390/larl-1.c
@@ -0,0 +1,32 @@
+/* Check if load-address-relative instructions are created */
+
+/* { dg-do compile { target { s390*-*-* } } } */
+/* { dg-options "-O2 -march=z10 -mzarch -fno-section-anchors" } */
+
+/* An explicitely misaligned symbol.  This symbol is NOT aligned as
+   mandated by our ABI.  However, the back-end needs to handle that in
+   order to make things like __attribute__((packed)) work.  The symbol
+   address is expected to be loaded from literal pool.  */
+/* { dg-final { scan-assembler "lgrl\t%r2," { target { lp64 } } } } */
+/* { dg-final { scan-assembler "lrl\t%r2," { target { ! lp64 } } } } */
+extern char align1 __attribute__((aligned(1)));
+
+/* { dg-final { scan-assembler "larl\t%r2,align2" } } */
+extern char align2 __attribute__((aligned(2)));
+
+/* { dg-final { scan-assembler "larl\t%r2,align4" } } */
+extern char align4 __attribute__((aligned(4)));
+
+/* An external char symbol without explicit alignment has a DECL_ALIGN
+   of just 8. In contrast to local definitions DATA_ABI_ALIGNMENT is
+   NOT applied to DECL_ALIGN in that case.  Make sure the backend
+   still assumes this symbol to be aligned according to ABI
+   requirements.  */
+/* { dg-final { scan-assembler "larl\t%r2,align_default" } } */
+extern char align_default;
+
+char * foo1 () { return &align1; }
+char * foo2 () { return &align2; }
+char * foo3 () { return &align4; }
+char * foo4 () { return &align_default; }
+