[gcc(refs/users/meissner/heads/work139)] PowerPC: Do not depend on an undefined shift

[Michael Meissner <meissner@gcc.gnu.org>]

https://gcc.gnu.org/g:5ba61cf9168a7f408a9af24d618a55ac44b3cb96

commit 5ba61cf9168a7f408a9af24d618a55ac44b3cb96
Author: Michael Meissner <meissner@linux.ibm.com>
Date:   Thu Oct 12 00:49:35 2023 -0400

    PowerPC: Do not depend on an undefined shift
    
    I was building a cross compiler to PowerPC on my x86_86 workstation with the
    latest version of GCC on October 11th.  I could not build the compiler on the
    x86_64 system as it died in building libgcc.  I looked into it, and I
    discovered the compiler was recursing until it ran out of stack space.  If I
    build a native compiler with the same sources on a PowerPC system, it builds
    fine.
    
    I traced this down to a change made around October 10th:
    
    | commit 8f1a70a4fbcc6441c70da60d4ef6db1e5635e18a (HEAD)
    | Author: Jiufu Guo <guojiufu@linux.ibm.com>
    | Date:   Tue Jan 10 20:52:33 2023 +0800
    |
    |   rs6000: build constant via li/lis;rldicl/rldicr
    |
    |   If a constant is possible left/right cleaned on a rotated value from
    |   a negative value of "li/lis".  Then, using "li/lis ; rldicl/rldicr"
    |   to build the constant.
    
    The code was doing a -1 << 64 which is undefined behavior because different
    machines produce different results.  On the x86_64 system, (-1 << 64) produces
    -1 while on a PowerPC 64-bit system, (-1 << 64) produces 0.  The x86_64 then
    recurses until the stack runs out of space.
    
    If I apply this patch, the compiler builds fine on both x86_64 as a PowerPC
    crosss compiler and on a native PowerPC system.
    
    2023-10-12  Michael Meissner  <meissner@linux.ibm.com>
    
    gcc/
    
            PR target/111778
            * config/rs6000/rs6000.cc (can_be_built_by_li_lis_and_rldicl): Protect
            code from shifts that are undefined.
            (can_be_built_by_li_lis_and_rldicr): Likewise.
            (can_be_built_by_li_and_rldic): Likewise.

Diff:
---
 gcc/config/rs6000/rs6000.cc | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
index 2d1d379e128..be6ae5c88bb 100644
--- a/gcc/config/rs6000/rs6000.cc
+++ b/gcc/config/rs6000/rs6000.cc
@@ -10371,6 +10371,7 @@ can_be_built_by_li_lis_and_rldicl (HOST_WIDE_INT c, int *shift,
      to ones and then recheck it.  */
   int lz = clz_hwi (c);
 <<<<<<< HEAD
+<<<<<<< HEAD
 
 <<<<<<< HEAD
   /* If lz == 0, the left shift is undefined.  */
@@ -10388,6 +10389,13 @@ can_be_built_by_li_lis_and_rldicl (HOST_WIDE_INT c, int *shift,
 
 =======
 >>>>>>> ab63444d033 (Revert patches)
+=======
+
+  /* If lz == 0, the left shift is undefined.  */
+  if (!lz)
+    return false;
+
+>>>>>>> 49fb3f85c90 (PowerPC: Do not depend on an undefined shift)
   HOST_WIDE_INT unmask_c
     = c | (HOST_WIDE_INT_M1U << (HOST_BITS_PER_WIDE_INT - lz));
   int n;