http://llvm.org/bugs/show_bug.cgi?id=20992
Bug ID: 20992
Summary: unexpected behavior using result of __lzcnt64() on
x86_64
Product: clang
Version: 3.4
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: -New Bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Created attachment 13050
--> http://llvm.org/bugs/attachment.cgi?id=13050&action=edit
test case compile with: clang -Wall -pedantic -O2 -mlzcnt -o lzcnt lzcntm.c
Given the following function definition
--
#include <stdint.h>
#include <x86intrin.h>
/*
* return position [0..63] of highest bit set or -1 if val equals zero
*/
int highest_bit_set(uint64_t val)
{
return 63 - __lzcnt64(val);
}
--
If called with val==0, the function returns 127 instead of -1.
Compiled on an x86_64 platform with
clang -Wall -pedantic -O2 -mlzcnt -S lzcnt.c
the compiler generates
-- clang 3.4 --
highest_bit_set: # @highest_bit_set
.cfi_startproc
# BB#0:
lzcntq %rdi, %rax
xorl $63, %eax
# kill: EAX<def> EAX<kill> RAX<kill>
ret
--
whereas gcc generates the expected
-- gcc 4.8.3 --
highest_bit_set:
.LFB551:
.cfi_startproc
lzcntq %rdi, %rdi
movl $63, %eax
subl %edi, %eax
ret
--
Disabling optimization yields the expected result. It seems the compiler
assumes __lzcnt64() will return only values [0..63] and replaces the 63-x by
xor $63,x.
The instruction lzcnt is specified to return the operand width in bits (64) if
the source operand is 0.
I am aware that lzcnt is a "problematic" instruction because it is executed as
bsr on processors with no native lzcnt support and the result of bsr is
undefined for a zero operand. For a cpu with native lzcnt support (-mlzcnt or
implicit via -march) i'd expected the compiler to support the full range of
values returned by lzcnt.
Related intrinsics/built-ins that (may) show the same problem: __tzcnt64(),
__builtin_clzl() ...
I first used __builtin_clzl() which translates into the same code, but which is
explicitly declared to return an undefined result (gcc docs) if called with a
zero argument.
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