https://llvm.org/bugs/show_bug.cgi?id=31755
Bug ID: 31755
Summary: carry detection missed optimization in chained
add-with-carry
Product: new-bugs
Version: 3.9
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: vincent-l...@vinc17.net
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
A carry can be detected so that an adcq instruction can be generated on x86_64.
But this does not work when the carry-out of this second addition is used for a
third addition. I have tested only on x86_64 (see below), but other similar
targets by be affected too.
Example:
typedef unsigned long T;
void f (T, T, T);
void add0 (T a, T b, T c, T w)
{
T u = a, v = b;
T carry1;
a = u + w;
carry1 = a < u;
b = v + carry1;
f (a, b, c);
}
void add1 (T a, T b, T c, T w)
{
T u = a, v = b;
T carry1, carry2;
a = u + w;
carry1 = a < u;
b = v + carry1;
carry2 = b < v;
c += carry2;
f (a, b, c);
}
Under Debian/unstable with Clang 3.9.1, using -O3, I get:
add0: # @add0
.cfi_startproc
# BB#0:
addq %rcx, %rdi
adcq $0, %rsi
jmp f # TAILCALL
Here, one can see that an adcq instruction is generated for the second
addition.
add1: # @add1
.cfi_startproc
# BB#0:
addq %rcx, %rdi
sbbq %rax, %rax
andl $1, %eax
addq %rax, %rsi
adcq $0, %rdx
jmp f # TAILCALL
The adcq instruction is generated for the third addition, but not for the
second one. The sbbq / andl / addq sequence could have been replaced by a
single adcq.
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