https://gcc.gnu.org/g:142b5263b18be96e5d9ce406ad2c1b6ab35c190f
commit r15-1751-g142b5263b18be96e5d9ce406ad2c1b6ab35c190f
Author: Roger Sayle <ro...@nextmovesoftware.com>
Date: Mon Jul 1 12:18:26 2024 +0100
i386: Additional peephole2 to use lea in round-up integer division.
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 both x and y are 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 recognize 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.
2024-07-01 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/ChangeLog
* gcc.target/i386/lea-3.c: New test case.
Diff:
---
gcc/config/i386/i386.md | 15 +++++++++++++++
gcc/testsuite/gcc.target/i386/lea-3.c | 13 +++++++++++++
2 files changed, 28 insertions(+)
diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index 59a889da304..0b6f6e75072 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
diff --git a/gcc/testsuite/gcc.target/i386/lea-3.c
b/gcc/testsuite/gcc.target/i386/lea-3.c
new file mode 100644
index 00000000000..84e66b00fc2
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/lea-3.c
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+int m;
+
+int foo(int y)
+{
+ return (m+y-1)/y;
+}
+
+/* { dg-final { scan-assembler "leal" } } */
+/* { dg-final { scan-assembler-not "addl" } } */
+/* { dg-final { scan-assembler-not "subl" } } */
