Re: [PATCH] tree-optimization: [PR103855] Fold (type)X / (type)Y

[Zhao Wei Liew <zhaoweiliew@gmail.com>]

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On Tue, 22 Feb 2022 at 11:57, Zhao Wei Liew <zhaoweiliew@gmail.com> wrote:
>
> Hi,
>
> This is a partial optimization for PR103855.
>
> Initially, I looked into optimizing RTL generation or a more complex
> GIMPLE transformation so that we could optimize other cases as well,
> such as ((unsigned long long) short / int).
>
> However, that is a bit too complex for now. While I continue to look
> into that change, I've decided to implement this simpler match.pd
> transformation.
>
> Greatly appreciate any feedback on this patch or guidance for
> implementing the more advanced optimizations!
>
> Thanks,
> Zhao Wei

Sorry, the original patch wasn't recognized as a text file. I've added
a .txt extension to make it explicit.

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From dd3bb05cd7be72d080598cb693549ac74d5cb02d Mon Sep 17 00:00:00 2001
From: Zhao Wei Liew <zhaoweiliew@gmail.com>
Date: Sat, 19 Feb 2022 16:28:38 +0800
Subject: [PATCH] tree-optimization: [PR103855] Fold (type)X / (type)Y

This pattern converts (trunc_div (convert a) (convert b)) to
(convert (trunc_div a b)) when:

1. type, a, and b all have unsigned integeral types
2. a and b have the same type precision
3. type has type precision at least as larger as a and b

This is useful as wider divisions are typically more expensive.

To illustrate the effects, consider the following code snippet:

unsigned long long f(unsigned int a, unsigned int b) {
	unsigned long long all = a;
	return all / b;
}

Without the pattern, g++ -std=c++20 -O2 generates the following
assembly:

f(unsigned int, unsigned int):
	mov eax, edi
	mov esi, esi
	xor edx, edx
	div rsi
	ret

With the pattern, it generates this:

f(unsigned int, unsigned int):
	mov eax, edi
	xor edx, edx
	div esi
	ret

This is identical to what clang++ -std=c++20 -O2 generates.

Bootstrapped and regression tested on x86_64-pc-linux-gnu.

Signed-off-by: Zhao Wei Liew <zhaoweiliew@gmail.com>

	PR tree-optimization/103855

gcc/ChangeLog:

	* match.pd: Add pattern for (type)X / (type)Y.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/divide-8.c: New test.
	* gcc.dg/tree-ssa/divide-9.c: New test.
---
 gcc/match.pd                             | 15 +++++++++++++++
 gcc/testsuite/gcc.dg/tree-ssa/divide-8.c |  9 +++++++++
 gcc/testsuite/gcc.dg/tree-ssa/divide-9.c |  9 +++++++++
 3 files changed, 33 insertions(+)
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/divide-8.c
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/divide-9.c

diff --git a/gcc/match.pd b/gcc/match.pd
index 10f62284862..393b43756dd 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -684,6 +684,21 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
  (if (INTEGRAL_TYPE_P (type) || VECTOR_INTEGER_TYPE_P (type))
   (convert (trunc_mod @0 @1))))
 
+/* (type)X / (type)Y -> (type)(X / Y)
+   when the resulting type is at least precise as the original types
+   and when all the types are unsigned integral types. */
+(simplify
+ (trunc_div (convert @0) (convert @1))
+ (if (INTEGRAL_TYPE_P (type)
+      && INTEGRAL_TYPE_P (TREE_TYPE (@0))
+      && INTEGRAL_TYPE_P (TREE_TYPE (@1))
+      && TYPE_UNSIGNED (type)
+      && TYPE_UNSIGNED (TREE_TYPE (@0))
+      && TYPE_UNSIGNED (TREE_TYPE (@1))
+      && TYPE_PRECISION (TREE_TYPE (@0)) == TYPE_PRECISION (TREE_TYPE (@1))
+      && TYPE_PRECISION (type) >= TYPE_PRECISION (TREE_TYPE (@0)))
+  (convert (trunc_div @0 @1))))
+
 /* x * (1 + y / x) - y -> x - y % x */
 (simplify
  (minus (mult:cs @0 (plus:s (trunc_div:s @1 @0) integer_onep)) @1)
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/divide-8.c b/gcc/testsuite/gcc.dg/tree-ssa/divide-8.c
new file mode 100644
index 00000000000..dc3dc9ca769
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/divide-8.c
@@ -0,0 +1,9 @@
+/* PR tree-optimization/103855 */
+/* { dg-options "-O -fdump-tree-optimized" } */
+
+unsigned int f(unsigned int a, unsigned int b) {
+    unsigned long long all = a;
+    return all / b;
+}
+
+/* { dg-final { scan-tree-dump-not "\\\(long long unsigned int\\\)" "optimized" } } */
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/divide-9.c b/gcc/testsuite/gcc.dg/tree-ssa/divide-9.c
new file mode 100644
index 00000000000..6986b5484e4
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/divide-9.c
@@ -0,0 +1,9 @@
+/* PR tree-optimization/103855 */
+/* { dg-options "-O -fdump-tree-optimized" } */
+
+unsigned long long f(unsigned int a, unsigned int b) {
+    unsigned long long all = a;
+    return all / b;
+}
+
+/* { dg-final { scan-tree-dump-times "\\\(long long unsigned int\\\)" 1 "optimized" } } */
-- 
2.35.1