On Sun, Oct 25, 2020 at 8:37 PM Stefan Kanthak <stefan.kant...@nexgo.de> wrote:
>
> Hi,
>
> for the AMD64 alias x86_64 platform and the __int128_t [DW]type,
> the first few lines of the __mulvDI3() function from libgcc2.c
>
> | DWtype
> | __mulvDI3 (DWtype u, DWtype v)
> | {
> | /* The unchecked multiplication needs 3 Wtype x Wtype multiplications,
> | but the checked multiplication needs only two. */
> | const DWunion uu = {.ll = u};
> | const DWunion vv = {.ll = v};
> |
> | if (__builtin_expect (uu.s.high == uu.s.low >> (W_TYPE_SIZE - 1), 1))
> | {
> | /* u fits in a single Wtype. */
> | if (__builtin_expect (vv.s.high == vv.s.low >> (W_TYPE_SIZE - 1), 1))
> | {
> | /* v fits in a single Wtype as well. */
> | /* A single multiplication. No overflow risk. */
> | return (DWtype) uu.s.low * (DWtype) vv.s.low;
> | }
>
> are compiled to this braindead code (obtained from libgcc.a of
> GCC 10.2.0 installed on Debian):
>
> 0000000000000000 <__mulvti3>:
> 0: 41 55 push %r13
> 2: 49 89 cb mov %rcx,%r11
> 5: 48 89 d0 mov %rdx,%rax
> 8: 49 89 d2 mov %rdx,%r10
> b: 41 54 push %r12
> d: 49 89 fc mov %rdi,%r12
> 10: 48 89 d1 mov %rdx,%rcx
> 13: 49 89 f0 mov %rsi,%r8
> 16: 4c 89 e2 mov %r12,%rdx
> 19: 49 89 f5 mov %rsi,%r13
> 1c: 53 push %rbx
> 1d: 48 89 fe mov %rdi,%rsi
> 20: 48 c1 fa 3f sar $0x3f,%rdx
> 24: 48 c1 f8 3f sar $0x3f,%rax
> 28: 4c 89 df mov %r11,%rdi
> 2b: 4c 39 c2 cmp %r8,%rdx
> 2e: 75 18 jne 48 <__mulvti3+0x48>
> 30: 4c 39 d8 cmp %r11,%rax
> 33: 75 6b jne a0 <__mulvti3+0xa0>
> 35: 4c 89 e0 mov %r12,%rax
> 38: 49 f7 ea imul %r10
> 3b: 5b pop %rbx
> 3c: 41 5c pop %r12
> 3e: 41 5d pop %r13
> 40: c3 retq
> ...
>
> There are EIGHT superfluous MOV instructions here, clobbering the
> non-volatile registers RBX, R12 and R13, plus THREE superfluous
> PUSH/POP pairs.
>
> What stops GCC from generating the following straightforward code
> (11 instructions in 31 bytes instead of 25 instructions in 65 bytes)?
>
> .intel_syntax noprefix
> __mulvti3:
> mov r8, rdi
> mov r9, rdx
> sra r8, 63
> sra r9, 63
> cmp r8, rsi
> jne __mulvti3+0x48+65-31
> cmp r9, rcx
> jne __mulvti3+0xa0+65-31
> mov rax, rdi
> imul rdx
> ret
> ...
>
>
> not amused
can you open a bugreport please?
Richard.
> Stefan Kanthak
