[pushed] rtl-ssa: Extend m_num_defs to a full unsigned int [PR108086]

[Richard Sandiford via Gcc-patches]

insn_info tried to save space by storing the number of
definitions in a 16-bit bitfield.  The justification was:

  // ...  FIRST_PSEUDO_REGISTER + 1
  // is the maximum number of accesses to hard registers and memory, and
  // MAX_RECOG_OPERANDS is the maximum number of pseudos that can be
  // defined by an instruction, so the number of definitions should fit
  // easily in 16 bits.

But while that reasoning holds (I think) for real instructions,
it doesn't hold for artificial instructions.  I don't think there's
any sensible higher limit we can use, so this patch goes for a full
unsigned int.

Tested on aarch64-linux-gnu.  Pushed (as obvious) to trunk so far,
will backport to GCC 12 and GCC 11 too.

Richard


gcc/
        PR rtl-optimization/108086
        * rtl-ssa/insns.h (insn_info): Make m_num_defs a full unsigned int.
        Adjust size-related commentary accordingly.
---
 gcc/rtl-ssa/insns.h | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/gcc/rtl-ssa/insns.h b/gcc/rtl-ssa/insns.h
index ffaf22d4b8e..a604fe295cd 100644
--- a/gcc/rtl-ssa/insns.h
+++ b/gcc/rtl-ssa/insns.h
@@ -141,7 +141,7 @@ using insn_call_clobbers_tree = 
default_splay_tree<insn_call_clobbers_note *>;
 // of "notes", a bit like REG_NOTES for the underlying RTL insns.
 class insn_info
 {
-  // Size: 8 LP64 words.
+  // Size: 9 LP64 words.
   friend class ebb_info;
   friend class function_info;
 
@@ -401,10 +401,11 @@ private:
   // The number of definitions and the number uses.  FIRST_PSEUDO_REGISTER + 1
   // is the maximum number of accesses to hard registers and memory, and
   // MAX_RECOG_OPERANDS is the maximum number of pseudos that can be
-  // defined by an instruction, so the number of definitions should fit
-  // easily in 16 bits.
+  // defined by an instruction, so the number of definitions in a real
+  // instruction should fit easily in 16 bits.  However, there are no
+  // limits on the number of definitions in artifical instructions.
   unsigned int m_num_uses;
-  unsigned int m_num_defs : 16;
+  unsigned int m_num_defs;
 
   // Flags returned by the accessors above.
   unsigned int m_is_debug_insn : 1;
@@ -414,7 +415,7 @@ private:
   unsigned int m_has_volatile_refs : 1;
 
   // For future expansion.
-  unsigned int m_spare : 11;
+  unsigned int m_spare : 27;
 
   // The program point at which the instruction occurs.
   //
@@ -431,6 +432,9 @@ private:
   // instruction.
   mutable int m_cost_or_uid;
 
+  // On LP64 systems, there's a gap here that could be used for future
+  // expansion.
+
   // The list of notes that have been attached to the instruction.
   insn_note *m_first_note;
 };
-- 
2.25.1