[x86 PATCH]: Additional peephole2 to use lea in round-up integer division.

[Roger Sayle]

A common idiom for implementing an integer division that rounds upwards is
to write (x + y - 1) / y.  Conveniently on x86, the two additions to form
the numerator can be performed by a single lea instruction, and indeed gcc
currently generates a lea when x and y both registers.

int foo(int x, int y) {
  return (x+y-1)/y;
}

generates with -O2:

foo:    leal    -1(%rsi,%rdi), %eax     // 4 bytes
        cltd
        idivl   %esi
        ret

Oddly, however, if x is a memory, gcc currently uses two instructions:

int m;
int bar(int y) {
  return (m+y-1)/y;
}

generates:

foo:    movl    m(%rip), %eax
        addl    %edi, %eax              // 2 bytes
        subl    $1, %eax                // 3 bytes
        cltd
        idivl   %edi
        ret

This discrepancy is caused by the late decision (in peephole2) to split
an addition with a memory operand, into a load followed by a reg-reg
addition.  This patch improves this situation by adding a peephole2
to recognized consecutive additions and transform them into lea if
profitable.

My first attempt at fixing this was to use a define_insn_and_split:

(define_insn_and_split "*lea<mode>3_reg_mem_imm"
  [(set (match_operand:SWI48 0 "register_operand")
       (plus:SWI48 (plus:SWI48 (match_operand:SWI48 1 "register_operand")
                               (match_operand:SWI48 2 "memory_operand"))
                   (match_operand:SWI48 3 "x86_64_immediate_operand")))]
  "ix86_pre_reload_split ()"
  "#"
  "&& 1"
  [(set (match_dup 4) (match_dup 2))
   (set (match_dup 0) (plus:SWI48 (plus:SWI48 (match_dup 1) (match_dup 4))
                                 (match_dup 3)))]
  "operands[4] = gen_reg_rtx (<MODE>mode);")

using combine to combine instructions.  Unfortunately, this approach
interferes with (reload's) subtle balance of deciding when to use/avoid lea,
which can be observed as a code size regression in CSiBE.  The peephole2
approach (proposed here) uniformly improves CSiBE results.

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?


2024-06-29  Roger Sayle  <ro...@nextmovesoftware.com>

gcc/ChangeLog
        * config/i386/i386.md (peephole2): Transform two consecutive
        additions into a 3-component lea if !TARGET_AVOID_LEA_FOR_ADDR.

gcc/testsuite/ChageLog
        * gcc.target/i386/lea-3.c: New test case.


Thanks in advance,
Roger
--


diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index fd48e76..66ef234 100644
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -6332,6 +6332,21 @@
   "TARGET_APX_NF && reload_completed"
   [(set (match_dup 0) (ashift:SWI48 (match_dup 0) (match_dup 1)))]
   "operands[1] = GEN_INT (exact_log2 (INTVAL (operands[1])));")
+
+;; The peephole2 pass may expose consecutive additions suitable for lea.
+(define_peephole2
+  [(parallel [(set (match_operand:SWI48 0 "register_operand")
+                  (plus:SWI48 (match_dup 0)
+                              (match_operand 1 "register_operand")))
+             (clobber (reg:CC FLAGS_REG))])
+   (parallel [(set (match_dup 0)
+                  (plus:SWI48 (match_dup 0)
+                              (match_operand 2 "x86_64_immediate_operand")))
+             (clobber (reg:CC FLAGS_REG))])]
+  "!TARGET_AVOID_LEA_FOR_ADDR || optimize_function_for_size_p (cfun)"
+  [(set (match_dup 0) (plus:SWI48 (plus:SWI48 (match_dup 0)
+                                             (match_dup 1))
+                                 (match_dup 2)))])
 
 ;; Add instructions