[gcc r11-9329] x86: Make stringop_algs::stringop_strategy ctor constexpr [PR100246]

[gcc r11-9329] x86: Make stringop_algs::stringop_strategy ctor constexpr [PR100246]

[Jakub Jelinek]

https://gcc.gnu.org/g:04d786a7add8f824af3b0c0d29884770810f7001

commit r11-9329-g04d786a7add8f824af3b0c0d29884770810f7001
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Nov 5 16:39:14 2021 +0100

    x86: Make stringop_algs::stringop_strategy ctor constexpr [PR100246]
    
    > Several older compilers fail to build modern GCC because of missing
    > or incomplete C++11 support.
    >
    >       * config/i386/i386.h (struct stringop_algs): Define a CTOR for
    >       this type.
    
    Unfortunately, as mentioned in my
    https://gcc.gnu.org/pipermail/gcc-patches/2021-November/583289.html
    mail, without the new dyninit pass this causes dynamic initialization of
    many variables, 6.5KB _GLOBAL__sub_I_* on x86_64 and 12.5KB on i686.
    
    The following patch makes the ctor constexpr so that already the FE
    is able to statically initialize all those.
    
    I have tested on godbolt a reduced testcase without a constructor,
    with constructor and with constexpr constructor.
    clang before 3.3 is unhappy about all the 3 cases, clang 3.3 and 3.4
    is ok with ctor and ctor with constexpr and optimizes it into static
    initialization, clang 3.5+ is ok with all 3 versions and optimizes,
    gcc 4.8 and 5+ is ok with all 3 versions and no ctor and ctor with constexpr
    is optimized, gcc 4.9 is unhappy about the no ctor case and happy with the
    other two.
    
    2021-11-05  Jakub Jelinek  <jakub@redhat.com>
    
            PR bootstrap/100246
            * config/i386/i386.h
            (stringop_algs::stringop_strategy::stringop_strategy): Make the ctor
            constexpr.
    
    (cherry picked from commit 858d7ee1a0cd97c01d844ea73ab81918da738344)

Diff:
---
 gcc/config/i386/i386.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/gcc/config/i386/i386.h b/gcc/config/i386/i386.h
index 73e118900f7..6fc1bf350d5 100644
--- a/gcc/config/i386/i386.h
+++ b/gcc/config/i386/i386.h
@@ -258,8 +258,9 @@ struct stringop_algs
        this issue.  Since this header is used by code compiled with the C
        compiler we must guard the addition.  */
 #ifdef __cplusplus
-    stringop_strategy(int _max = -1, enum stringop_alg _alg = libcall,
-		      int _noalign = false)
+    constexpr
+    stringop_strategy (int _max = -1, enum stringop_alg _alg = libcall,
+		       int _noalign = false)
       : max (_max), alg (_alg), noalign (_noalign) {}
 #endif
     const int max;