[PATCH] New finish_compare_by_pieces target hook (for x86).

["Roger Sayle" <roger@nextmovesoftware.com>]

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The following simple test case, from PR 104610, shows that memcmp () == 0
can result in some bizarre code sequences on x86.

int foo(char *a)
{
    static const char t[] = "0123456789012345678901234567890";
    return __builtin_memcmp(a, &t[0], sizeof(t)) == 0;
}

with -O2 currently contains both:
        xorl    %eax, %eax
        xorl    $1, %eax
and also
        movl    $1, %eax
        xorl    $1, %eax

Changing the return type of foo to _Bool results in the equally
bizarre:
        xorl    %eax, %eax
        testl   %eax, %eax
        sete    %al
and also
        movl    $1, %eax
        testl   %eax, %eax
        sete    %al

All these sequences set the result to a constant, but this optimization
opportunity only occurs very late during compilation, by basic block
duplication in the 322r.bbro pass, too late for CSE or peephole2 to
do anything about it.  The problem is that the idiom expanded by
compare_by_pieces for __builtin_memcmp_eq contains basic blocks that
can't easily be optimized by if-conversion due to the multiple
incoming edges on the fail block.

In summary, compare_by_pieces generates code that looks like:

        if (x[0] != y[0]) goto fail_label;
        if (x[1] != y[1]) goto fail_label;
        ...
        if (x[n] != y[n]) goto fail_label;
        result = 1;
        goto end_label;
fail_label:
        result = 0;
end_label:

In theory, the RTL if-conversion pass could be enhanced to tackle
arbitrarily complex if-then-else graphs, but the solution proposed
here is to allow suitable targets to perform if-conversion during
compare_by_pieces.  The x86, for example, can take advantage that
all of the above comparisons set and test the zero flag (ZF), which
can then be used in combination with sete.  Hence compare_by_pieces
could instead generate:

        if (x[0] != y[0]) goto fail_label;
        if (x[1] != y[1]) goto fail_label;
        ...
        if (x[n] != y[n]) goto fail_label;
fail_label:
        sete result

which requires one less basic block, and the redundant conditional
branch to a label immediately after is cleaned up by GCC's existing
RTL optimizations.

For the test case above, where -O2 -msse4 previously generated:

foo:    movdqu  (%rdi), %xmm0
        pxor    .LC0(%rip), %xmm0
        ptest   %xmm0, %xmm0
        je      .L5
.L2:    movl    $1, %eax
        xorl    $1, %eax
        ret
.L5:    movdqu  16(%rdi), %xmm0
        pxor    .LC1(%rip), %xmm0
        ptest   %xmm0, %xmm0
        jne     .L2
        xorl    %eax, %eax
        xorl    $1, %eax
        ret

we now generate:

foo:    movdqu  (%rdi), %xmm0
        pxor    .LC0(%rip), %xmm0
        ptest   %xmm0, %xmm0
        jne     .L2
        movdqu  16(%rdi), %xmm0
        pxor    .LC1(%rip), %xmm0
        ptest   %xmm0, %xmm0
.L2:    sete    %al
        movzbl  %al, %eax
        ret

Using a target hook allows the large amount of intelligence already in
compare_by_pieces to be re-used by the i386 backend, but this can also
help other backends with condition flags where the equality result can
be materialized.

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}
with no new failures.  Ok for mainline?


2023-06-12  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        * config/i386/i386.cc (ix86_finish_compare_by_pieces): New
        function to provide a backend specific implementation.
        (TARGET_FINISH_COMPARE_BY_PIECES): Use the above function.

        * doc/tm.texi.in (TARGET_FINISH_COMPARE_BY_PIECES): New @hook.
        * doc/tm.texi: Regenerate.

        * expr.cc (compare_by_pieces): Call finish_compare_by_pieces in
        targetm to finalize the RTL expansion.  Move the current
        implementation to a default target hook.
        * target.def (finish_compare_by_pieces): New target hook to allow
        compare_by_pieces to be customized by the target.
        * targhooks.cc (default_finish_compare_by_pieces): Default
        implementation moved here from expr.cc's compare_by_pieces.
        * targhooks.h (default_finish_compare_by_pieces): Prototype.

gcc/testsuite/ChangeLog
        * gcc.target/i386/pieces-memcmp-1.c: New test case.


Thanks in advance,
Roger
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diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index 3a1444d..509c0ee 100644
--- a/gcc/config/i386/i386.cc
+++ b/gcc/config/i386/i386.cc
@@ -16146,6 +16146,20 @@ ix86_fp_compare_code_to_integer (enum rtx_code code)
     }
 }
 
+/* Override compare_by_pieces' default implementation using the state
+   of the CCZmode FLAGS_REG and sete instruction.  TARGET is the integral
+   mode result, and FAIL_LABEL is the branch target of mismatched
+   comparisons.  */
+
+void
+ix86_finish_compare_by_pieces (rtx target, rtx_code_label *fail_label)
+{
+  rtx tmp = gen_reg_rtx (QImode);
+  emit_label (fail_label);
+  ix86_expand_setcc (tmp, NE, gen_rtx_REG (CCZmode, FLAGS_REG), const0_rtx);
+  convert_move (target, tmp, 1);
+}
+
 /* Zero extend possibly SImode EXP to Pmode register.  */
 rtx
 ix86_zero_extend_to_Pmode (rtx exp)
@@ -25127,6 +25141,8 @@ ix86_run_selftests (void)
 
 #undef TARGET_OVERLAP_OP_BY_PIECES_P
 #define TARGET_OVERLAP_OP_BY_PIECES_P hook_bool_void_true
+#undef TARGET_FINISH_COMPARE_BY_PIECES
+#define TARGET_FINISH_COMPARE_BY_PIECES ix86_finish_compare_by_pieces
 
 #undef TARGET_FLAGS_REGNUM
 #define TARGET_FLAGS_REGNUM FLAGS_REG
diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index 95ba56e..28d8361 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -6922,6 +6922,18 @@ particular mode from being used for block comparisons by returning a
 negative number from this hook.
 @end deftypefn
 
+@deftypefn {Target Hook} void TARGET_FINISH_COMPARE_BY_PIECES (rtx @var{target}, rtx_code_label *@var{fail_label})
+Allow targets with a zero flag and suitable setcc instruction to provide
+an alternate implementation for @code{compare_by_pieces}.  The function
+@code{compare_by_pieces} generates a sequence of equality tests that
+branch to @var{failure_label} on mismatches, and fall through on success.
+By default, this hook assigns @code{const1_rtx} to @var{target} in the
+current basic block and @code{const0_rtx} to @var{target} in a new
+@var{fail_label} basic block.  Targets like x86 can take advantage
+of the property that the condition codes/zero flag are appropriately
+set to avoid introducing a new basic block.
+@end deftypefn
+
 @defmac MOVE_MAX_PIECES
 A C expression used by @code{move_by_pieces} to determine the largest unit
 a load or store used to copy memory is.  Defaults to @code{MOVE_MAX}.
diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
index 4ac96dc..45711db 100644
--- a/gcc/doc/tm.texi.in
+++ b/gcc/doc/tm.texi.in
@@ -4529,6 +4529,8 @@ If you don't define this, a reasonable default is used.
 
 @hook TARGET_COMPARE_BY_PIECES_BRANCH_RATIO
 
+@hook TARGET_FINISH_COMPARE_BY_PIECES
+
 @defmac MOVE_MAX_PIECES
 A C expression used by @code{move_by_pieces} to determine the largest unit
 a load or store used to copy memory is.  Defaults to @code{MOVE_MAX}.
diff --git a/gcc/expr.cc b/gcc/expr.cc
index 868fa6e..25fca17 100644
--- a/gcc/expr.cc
+++ b/gcc/expr.cc
@@ -1923,7 +1923,6 @@ compare_by_pieces (rtx arg0, rtx arg1, unsigned HOST_WIDE_INT len,
 		   by_pieces_constfn a1_cfn, void *a1_cfn_data)
 {
   rtx_code_label *fail_label = gen_label_rtx ();
-  rtx_code_label *end_label = gen_label_rtx ();
 
   if (target == NULL_RTX
       || !REG_P (target) || REGNO (target) < FIRST_PSEUDO_REGISTER)
@@ -1934,12 +1933,8 @@ compare_by_pieces (rtx arg0, rtx arg1, unsigned HOST_WIDE_INT len,
 
   data.run ();
 
-  emit_move_insn (target, const0_rtx);
-  emit_jump (end_label);
-  emit_barrier ();
-  emit_label (fail_label);
-  emit_move_insn (target, const1_rtx);
-  emit_label (end_label);
+  /* Allow the backend to override the default implementation.  */
+  targetm.finish_compare_by_pieces (target, fail_label);
 
   return target;
 }
diff --git a/gcc/target.def b/gcc/target.def
index 7d68429..0593917 100644
--- a/gcc/target.def
+++ b/gcc/target.def
@@ -3749,6 +3749,20 @@ negative number from this hook.",
  default_compare_by_pieces_branch_ratio)
 
 DEFHOOK
+(finish_compare_by_pieces,
+ "Allow targets with a zero flag and suitable setcc instruction to provide\n\
+an alternate implementation for @code{compare_by_pieces}.  The function\n\
+@code{compare_by_pieces} generates a sequence of equality tests that\n\
+branch to @var{failure_label} on mismatches, and fall through on success.\n\
+By default, this hook assigns @code{const1_rtx} to @var{target} in the\n\
+current basic block and @code{const0_rtx} to @var{target} in a new\n\
+@var{fail_label} basic block.  Targets like x86 can take advantage\n\
+of the property that the condition codes/zero flag are appropriately\n\
+set to avoid introducing a new basic block.",
+  void, (rtx target, rtx_code_label *fail_label),
+  default_finish_compare_by_pieces)
+
+DEFHOOK
 (slow_unaligned_access,
  "This hook returns true if memory accesses described by the\n\
 @var{mode} and @var{alignment} parameters have a cost many times greater\n\
diff --git a/gcc/targhooks.cc b/gcc/targhooks.cc
index e190369..767fe38 100644
--- a/gcc/targhooks.cc
+++ b/gcc/targhooks.cc
@@ -2094,6 +2094,24 @@ default_compare_by_pieces_branch_ratio (machine_mode)
   return 1;
 }
 
+/* This hook allows the backend to modify/override code generation for
+   compare_by_pieces.  Targets with a zero flag and a suitable setcc
+   function can use them instead of the default "compare ? 0 : 1"
+   implementation below.  TARGET is the integral mode result and
+   FAIL_LABEL is the destination for comparison mismatches.  */
+
+void
+default_finish_compare_by_pieces (rtx target, rtx_code_label *fail_label)
+{
+  rtx_code_label *end_label = gen_label_rtx ();
+  emit_move_insn (target, const0_rtx);
+  emit_jump (end_label);
+  emit_barrier ();
+  emit_label (fail_label);
+  emit_move_insn (target, const1_rtx);
+  emit_label (end_label);
+}
+
 /* Write PATCH_AREA_SIZE NOPs into the asm outfile FILE around a function
    entry.  If RECORD_P is true and the target supports named sections,
    the location of the NOPs will be recorded in a special object section
diff --git a/gcc/targhooks.h b/gcc/targhooks.h
index 1a0db8d..b99fff6 100644
--- a/gcc/targhooks.h
+++ b/gcc/targhooks.h
@@ -237,6 +237,7 @@ extern bool default_use_by_pieces_infrastructure_p (unsigned HOST_WIDE_INT,
 						    enum by_pieces_operation,
 						    bool);
 extern int default_compare_by_pieces_branch_ratio (machine_mode);
+extern void default_finish_compare_by_pieces (rtx, rtx_code_label *);
 
 extern void default_print_patchable_function_entry (FILE *,
 						    unsigned HOST_WIDE_INT,
diff --git a/gcc/testsuite/gcc.target/i386/pieces-memcmp-1.c b/gcc/testsuite/gcc.target/i386/pieces-memcmp-1.c
new file mode 100644
index 0000000..de1d82f
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pieces-memcmp-1.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+int foo(char *a)
+{
+    static const char t[] = "0123456789012345678901234567890";
+    return __builtin_memcmp(a, &t[0], sizeof(t)) == 0;
+}
+
+/* { dg-final { scan-assembler-not "xorl\[ \\t]*\\\$1," } } */