Consider the following functions:
typedef unsigned long long int u64;
void foo(u64* d, u64 const* s, u64 k) {
*d = ((__uint128_t) *s*k) >> 64;
}
void foo(u64* d, u64 const* s, u64 k, u64 m) {
*d = ((__uint128_t) (*s&m)*k) >> 64;
}
void foo2(u64* d, u64 const* s, u64 k) {
foo(d, s, k);
foo(d+1,s+1,k);
}
void foo2(u64* d, u64 const* s, u64 k, u64 m) {
foo(d, s, k, m);
foo(d+1,s+1,k, m);
}
Compiling them with "g++ -O3" gives:
_Z3fooPyPKyy:
movq %rdx, %rax
mulq (%rsi)
movq %rdx, (%rdi)
ret
_Z3fooPyPKyyy:
andq (%rsi), %rcx
movq %rcx, %rax
mulq %rdx
movq %rdx, (%rdi)
ret
_Z4foo2PyPKyy:
movq (%rsi), %rax
xorl %r9d, %r9d
movq %rdx, %r8
movq %r9, %rcx
imulq %rax, %rcx
mulq %rdx
leaq (%rcx,%rdx), %rdx
movq %r9, %rcx
movq %rdx, (%rdi)
movq 8(%rsi), %rax
imulq %rax, %rcx
mulq %r8
leaq (%rcx,%rdx), %rdx
movq %rdx, 8(%rdi)
ret
_Z4foo2PyPKyyy:
movq %rcx, %rax
andq (%rsi), %rax
movq %rdx, %r10
xorl %r11d, %r11d
xorl %edx, %edx
movq %rdx, %r8
movq %r11, %r9
imulq %r10, %r8
imulq %rax, %r9
mulq %r10
addq %r9, %r8
leaq (%r8,%rdx), %rdx
movq %rdx, (%rdi)
andq 8(%rsi), %rcx
xorl %edx, %edx
movq %r11, %rsi
movq %rcx, %rax
movq %rdx, %rcx
imulq %rax, %rsi
imulq %r10, %rcx
mulq %r10
addq %rsi, %rcx
leaq (%rcx,%rdx), %rdx
movq %rdx, 8(%rdi)
ret
The two versions of foo() do exactly what you would expect: AND+MUL, then store
the high dword. The two versions of foo2(), on the other hand, perform two and
four signed multiplies, in addition to the two unsigned multiplies that would
be expected. In my debugger, at least, "xorl %edx, %edx" zeros out all 64 bits,
so the two signed multiplies give zero for their result, making them completely
redundant.
Compiling without optimizations gives the IMUL+IMUL+MUL combination even for
foo(), so it appears that the optimizer is missing something once it has more
than one multiply to deal with.
--
Summary: Significant extra code generation for 64x64=>128-bit
multiply
Product: gcc
Version: 4.1.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: scovich at gmail dot com
GCC target triplet: x86_64-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32662
