Date: Wed, 1 Jul 2026 19:32:16 -0500
From: Nathan Bossart <nathandbossart@gmail.com>
To: pgsql-hackers@postgresql.org
Cc: johncnaylorls@gmail.com
Subject: remove switch statement in vector8_shift_{left,right}
Message-ID: <akWxkA-mszMm57cV@nathan>
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In commit ec8719ccbf, I added a switch statement with all expected shift
counts for vector8_shift_{left,right} because AArch64's vshlq_n_u32 and
vshrq_n_u32 require an integer literal.  I discovered that we can use
vshlq_u32 instead for both cases, which avoids the need for the switch
statement and compiles to the same machine code on recent gcc/clang.

-- 
nathan

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	filename=v1-0001-remove-switch-statement-in-vector8_shift_-left-ri.patch

From 12b9db50eed370ba94ed4a7c4c9dd2218bad5f48 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Wed, 1 Jul 2026 19:30:59 -0500
Subject: [PATCH v1 1/1] remove switch statement in vector8_shift_{left,right}

---
 src/include/port/simd.h | 29 +++--------------------------
 1 file changed, 3 insertions(+), 26 deletions(-)

diff --git a/src/include/port/simd.h b/src/include/port/simd.h
index 50615aec7f4..25a26c81cce 100644
--- a/src/include/port/simd.h
+++ b/src/include/port/simd.h
@@ -548,10 +548,6 @@ vector8_pack_16(const Vector8 v1, const Vector8 v2)
 
 /*
  * Unsigned shift left of each 32-bit element in the vector by "i" bits.
- *
- * XXX AArch64 requires an integer literal, so we have to list all expected
- * values of "i" from all callers in a switch statement.  If you add a new
- * caller, be sure your expected values of "i" are handled.
  */
 #ifndef USE_NO_SIMD
 static inline Vector8
@@ -560,24 +556,13 @@ vector8_shift_left(const Vector8 v1, int i)
 #ifdef USE_SSE2
 	return _mm_slli_epi32(v1, i);
 #elif defined(USE_NEON)
-	switch (i)
-	{
-		case 4:
-			return (Vector8) vshlq_n_u32((Vector32) v1, 4);
-		default:
-			Assert(false);
-			return vector8_broadcast(0);
-	}
+	return (Vector8) vshlq_u32((Vector32) v1, vdupq_n_s32(i));
 #endif
 }
 #endif							/* ! USE_NO_SIMD */
 
 /*
  * Unsigned shift right of each 32-bit element in the vector by "i" bits.
- *
- * XXX AArch64 requires an integer literal, so we have to list all expected
- * values of "i" from all callers in a switch statement.  If you add a new
- * caller, be sure your expected values of "i" are handled.
  */
 #ifndef USE_NO_SIMD
 static inline Vector8
@@ -586,16 +571,8 @@ vector8_shift_right(const Vector8 v1, int i)
 #ifdef USE_SSE2
 	return _mm_srli_epi32(v1, i);
 #elif defined(USE_NEON)
-	switch (i)
-	{
-		case 4:
-			return (Vector8) vshrq_n_u32((Vector32) v1, 4);
-		case 8:
-			return (Vector8) vshrq_n_u32((Vector32) v1, 8);
-		default:
-			Assert(false);
-			return vector8_broadcast(0);
-	}
+	/* negative shift count means right shift */
+	return (Vector8) vshlq_u32((Vector32) v1, vdupq_n_s32(-i));
 #endif
 }
 #endif							/* ! USE_NO_SIMD */
-- 
2.50.1 (Apple Git-155)


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