http://llvm.org/bugs/show_bug.cgi?id=17128
Bug ID: 17128
Summary: bit-scan-forward / count-trailing-zeros loop not
recognized
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
$ ./clang -v
clang version 3.4 (trunk 189776)
Target: x86_64-apple-darwin11.4.2
Thread model: posix
$ cat tzcnt.c
int tzcnt(int x) {
int count = 0;
int i = 0;
while ( i<32 && (((x >> i) & 0x1) == 0)) {
i++;
count++;
}
return count;
}
$ ./clang -S -O3 -fomit-frame-pointer -march=core-avx2 -o /dev/stdout tzcnt.c
.section __TEXT,__text,regular,pure_instructions
.globl _tzcnt
.align 4, 0x90
_tzcnt: ## @tzcnt
.cfi_startproc
## BB#0: ## %entry
xorl %eax, %eax
.align 4, 0x90
LBB0_1: ## %land.rhs
## =>This Inner Loop Header: Depth=1
btl %eax, %edi
jb LBB0_3
## BB#2: ## %while.body
## in Loop: Header=BB0_1 Depth=1
incl %eax
cmpl $32, %eax
jl LBB0_1
LBB0_3: ## %while.end
ret
...
On CPUs with the BMI feature, I was hoping this loop would generate the 'tzcnt'
instruction:
tzcnt %edi, %eax
On x86 CPUs without BMI, this loop could also be implemented with the 'bsf'
instruction with a leading check for a zero input value. In the case of zero
input, the compiler would have to return '32' because the hardware doesn't.
According to Intel's Volume 2 ISA reference:
"The key difference between TZCNT and BSF instruction is that TZCNT provides
operand size as output when source operand is zero while in the case of BSF
instruction, if source operand is zero, the content of destination operand are
undefined. On processors that do not support TZCNT, the instruction byte
encoding is executed as BSF."
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