acl_hash: fix integer conversion warning

[pcolberg]

Avoid integer conversion altogether by using a more efficient
implementation which compiles to branchless code with a rotate
instruction, assuming the number of bits to rotate by is known
at compile time so the compiler may eliminate the assert().

[pcolberg]

Current version of bitwise left-rotate:

// rotate_old.c 
#include <stdint.h>
uint32_t l_leftrotate(uint32_t v, unsigned bits) {
  uint64_t both = ((uint64_t)v) << bits;
  uint32_t hi = both >> 32;
  uint32_t lo = both & 0xffffffff;
  return hi | lo;
}

Improved version of bitwise left-rotate:

// rotate.c
#include <assert.h>
#include <stdint.h>
// See Safe, Efficient, and Portable Rotate in C/C++ by John Regehr.
// https://blog.regehr.org/archives/1063
uint32_t l_leftrotate(uint32_t v, uint32_t bits) {
  assert(bits < 32);
  return (v << bits) | (v >> ((-bits) & 31));
}

Comparison with GCC 10.2.1, improved version emits rotate instruction:

--- rotate_old.s
+++ rotate.s
@@ -1,20 +1,17 @@
-	.file	"rotate_old.c"
+	.file	"rotate.c"
 	.text
 	.p2align 4
 	.globl	l_leftrotate
 	.type	l_leftrotate, @function
 l_leftrotate:
 .LFB0:
 	.cfi_startproc
 	movl	%edi, %eax
 	movl	%esi, %ecx
-	salq	%cl, %rax
-	movq	%rax, %rdi
-	shrq	$32, %rdi
-	orl	%edi, %eax
+	roll	%cl, %eax
 	ret
 	.cfi_endproc
 .LFE0:
 	.size	l_leftrotate, .-l_leftrotate
 	.ident	"GCC: (Debian 10.2.1-6) 10.2.1 20210110"
 	.section	.note.GNU-stack,"",@progbits

Comparison with Clang 11.0.1, improved version emits rotate instruction:

--- rotate_old.s
+++ rotate.s
@@ -1,24 +1,20 @@
 	.text
-	.file	"rotate_old.c"
+	.file	"rotate.c"
 	.globl	l_leftrotate                    # -- Begin function l_leftrotate
 	.p2align	4, 0x90
 	.type	l_leftrotate,@function
 l_leftrotate:                           # @l_leftrotate
 	.cfi_startproc
 # %bb.0:
 	movl	%esi, %ecx
-	movl	%edi, %edx
+	movl	%edi, %eax
                                         # kill: def $cl killed $cl killed $ecx
-	shlq	%cl, %rdx
-	movq	%rdx, %rax
-	shrq	$32, %rax
-	orl	%edx, %eax
-                                        # kill: def $eax killed $eax killed $rax
+	roll	%cl, %eax
 	retq
 .Lfunc_end0:
 	.size	l_leftrotate, .Lfunc_end0-l_leftrotate
 	.cfi_endproc
                                         # -- End function
 	.ident	"Debian clang version 11.0.1-2"
 	.section	".note.GNU-stack","",@progbits
 	.addrsig

[pcolberg]

Via Best practices for circular shift (rotate) operations in C++.

[pcolberg]

MSVC on Windows fails with

acl_hash.c(220): error C4146: unary minus operator applied to unsigned type, result still unsigned

fpga-runtime-for-opencl/lib/acl_hash/src/acl_hash.c

Lines 216 to 221 in 7e23a82

	// See Safe, Efficient, and Portable Rotate in C/C++ by John Regehr.
	// https://blog.regehr.org/archives/1063
	static uint32_t l_leftrotate(uint32_t v, uint32_t bits) {
	assert(bits < 32);
	return (v << bits) | (v >> ((-bits) & 31));
	}

[zibaiwan]

Thanks @pcolberg for fixing this!