From: Daniel Engel <g...@danielengel.com>
The functional overlap between the single- and double-word versions
functions makes this implementation about 30% smaller than C when
both functions are linked together in an appliation.
gcc/libgcc/ChangeLog:
2021-01-07 Daniel Engel <g...@danielengel.com>
* config/arm/bits/popcnt.S: New file for __popcountsi2/di2()..
* config/arm/lib1funcs.S: #include bit/popcnt.S.
* config/arm/t-elf: Add _popcount* objects to LIB1ASMFUNCS.
---
libgcc/config/arm/bits/popcnt.S | 189 ++++++++++++++++++++++++++++++++
libgcc/config/arm/lib1funcs.S | 1 +
libgcc/config/arm/t-elf | 2 +
3 files changed, 192 insertions(+)
create mode 100644 libgcc/config/arm/bits/popcnt.S
diff --git a/libgcc/config/arm/bits/popcnt.S b/libgcc/config/arm/bits/popcnt.S
new file mode 100644
index 00000000000..13642267d64
--- /dev/null
+++ b/libgcc/config/arm/bits/popcnt.S
@@ -0,0 +1,189 @@
+/* popcnt.S: ARM optimized popcount functions
+
+ Copyright (C) 2020-2021 Free Software Foundation, Inc.
+ Contributed by Daniel Engel (g...@danielengel.com)
+
+ This file is free software; you can redistribute it and/or modify it
+ under the terms of the GNU General Public License as published by the
+ Free Software Foundation; either version 3, or (at your option) any
+ later version.
+
+ This file is distributed in the hope that it will be useful, but
+ WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ General Public License for more details.
+
+ Under Section 7 of GPL version 3, you are granted additional
+ permissions described in the GCC Runtime Library Exception, version
+ 3.1, as published by the Free Software Foundation.
+
+ You should have received a copy of the GNU General Public License and
+ a copy of the GCC Runtime Library Exception along with this program;
+ see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
+ <http://www.gnu.org/licenses/>. */
+
+
+#ifdef L_popcountdi2
+
+// int __popcountdi2(int)
+// Returns the number of bits set in $r1:$r0.
+// Returns the result in $r0.
+FUNC_START_SECTION popcountdi2 .text.sorted.libgcc.popcountdi2
+ CFI_START_FUNCTION
+
+ #if defined(__OPTIMIZE_SIZE__) && __OPTIMIZE_SIZE__
+ // Initialize the result.
+ // Compensate for the two extra loop (one for each word)
+ // required to detect zero arguments.
+ movs r2, #2
+
+ LLSYM(__popcountd_loop):
+ // Same as __popcounts_loop below, except for $r1.
+ subs r2, #1
+ subs r3, r1, #1
+ ands r1, r3
+ bcs LLSYM(__popcountd_loop)
+
+ // Repeat the operation for the second word.
+ b LLSYM(__popcounts_loop)
+
+ #else /* !__OPTIMIZE_SIZE__ */
+ // Load the one-bit alternating mask.
+ ldr r3, =0x55555555
+
+ // Reduce the second word.
+ lsrs r2, r1, #1
+ ands r2, r3
+ subs r1, r2
+
+ // Reduce the first word.
+ lsrs r2, r0, #1
+ ands r2, r3
+ subs r0, r2
+
+ // Load the two-bit alternating mask.
+ ldr r3, =0x33333333
+
+ // Reduce the second word.
+ lsrs r2, r1, #2
+ ands r2, r3
+ ands r1, r3
+ adds r1, r2
+
+ // Reduce the first word.
+ lsrs r2, r0, #2
+ ands r2, r3
+ ands r0, r3
+ adds r0, r2
+
+ // There will be a maximum of 8 bits in each 4-bit field.
+ // Jump into the single word flow to combine and complete.
+ b LLSYM(__popcounts_merge)
+
+ #endif /* !__OPTIMIZE_SIZE__ */
+#endif /* L_popcountdi2 */
+
+
+// The implementation of __popcountdi2() tightly couples with __popcountsi2(),
+// such that instructions must appear consecutively in the same memory
+// section for proper flow control. However, this construction inhibits
+// the ability to discard __popcountdi2() when only using __popcountsi2().
+// Therefore, this block configures __popcountsi2() for compilation twice.
+// The first version is a minimal standalone implementation, and the second
+// version is the continuation of __popcountdi2(). The standalone version
must
+// be declared WEAK, so that the combined version can supersede it and
+// provide both symbols when required.
+// '_popcountsi2' should appear before '_popcountdi2' in LIB1ASMFUNCS.
+#if defined(L_popcountsi2) || defined(L_popcountdi2)
+
+#ifdef L_popcountsi2
+// int __popcountsi2(int)
+// Returns '0' if the number of bits set in $r0 is even, and '1' otherwise.
+// Returns the result in $r0.
+// Uses $r2 as scratch space.
+WEAK_START_SECTION popcountsi2 .text.sorted.libgcc.popcountsi2
+ CFI_START_FUNCTION
+
+#else /* L_popcountdi2 */
+FUNC_ENTRY popcountsi2
+
+#endif
+
+ #if defined(__OPTIMIZE_SIZE__) && __OPTIMIZE_SIZE__
+ // Initialize the result.
+ // Compensate for the extra loop required to detect zero.
+ movs r2, #1
+
+ // Kernighan's algorithm for __popcount(x):
+ // for (c = 0; x; c++)
+ // x &= x - 1;
+
+ LLSYM(__popcounts_loop):
+ // Every loop counts for a '1' set in the argument.
+ // Count down since it's easier to initialize positive compensation,
+ // and the negation before function return is free.
+ subs r2, #1
+
+ // Clear one bit per loop.
+ subs r3, r0, #1
+ ands r0, r3
+
+ // If this is a test for zero, it will be impossible to distinguish
+ // between zero and one bits set: both terminate after one loop.
+ // Instead, subtraction underflow flags when zero entered the loop.
+ bcs LLSYM(__popcounts_loop)
+
+ // Invert the result, since we have been counting negative.
+ rsbs r0, r2, #0
+ RET
+
+ #else /* !__OPTIMIZE_SIZE__ */
+
+ // Load the one-bit alternating mask.
+ ldr r3, =0x55555555
+
+ // Reduce the word.
+ lsrs r1, r0, #1
+ ands r1, r3
+ subs r0, r1
+
+ // Load the two-bit alternating mask.
+ ldr r3, =0x33333333
+
+ // Reduce the word.
+ lsrs r1, r0, #2
+ ands r0, r3
+ ands r1, r3
+ LLSYM(__popcounts_merge):
+ adds r0, r1
+
+ // Load the four-bit alternating mask.
+ ldr r3, =0x0F0F0F0F
+
+ // Reduce the word.
+ lsrs r1, r0, #4
+ ands r0, r3
+ ands r1, r3
+ adds r0, r1
+
+ // Accumulate individual byte sums into the MSB.
+ lsls r1, r0, #8
+ adds r0, r1
+ lsls r1, r0, #16
+ adds r0, r1
+
+ // Isolate the cumulative sum.
+ lsrs r0, #24
+ RET
+
+ #endif /* !__OPTIMIZE_SIZE__ */
+
+ CFI_END_FUNCTION
+FUNC_END popcountsi2
+
+#ifdef L_popcountdi2
+FUNC_END popcountdi2
+#endif
+
+#endif /* L_popcountsi2 || L_popcountdi2 */
+
diff --git a/libgcc/config/arm/lib1funcs.S b/libgcc/config/arm/lib1funcs.S
index a05c0bc1c55..2323fefa731 100644
--- a/libgcc/config/arm/lib1funcs.S
+++ b/libgcc/config/arm/lib1funcs.S
@@ -1639,6 +1639,7 @@ LSYM(Lover12):
#include "bits/clz2.S"
#include "bits/ctz2.S"
#include "bits/parity.S"
+#include "bits/popcnt.S"
/* ------------------------------------------------------------------------ */
/* These next two sections are here despite the fact that they contain Thumb
diff --git a/libgcc/config/arm/t-elf b/libgcc/config/arm/t-elf
index 22fb00e4ca4..c853b543419 100644
--- a/libgcc/config/arm/t-elf
+++ b/libgcc/config/arm/t-elf
@@ -26,6 +26,7 @@ LIB1ASMFUNCS += \
_clzsi2 \
_ctzsi2 \
_paritysi2 \
+ _popcountsi2 \
# Group 1: Integer functions
@@ -40,6 +41,7 @@ LIB1ASMFUNCS += \
_ffssi2 \
_ffsdi2 \
_paritydi2 \
+ _popcountdi2 \
_dvmd_tls \
_divsi3 \
_modsi3 \
--
2.25.1
