[Bug c/65436] New: Max number of extended asm +input operands currently limited to 15

[adam at consulting dot net.nz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65436

            Bug ID: 65436
           Summary: Max number of extended asm +input operands currently
                    limited to 15
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: adam at consulting dot net.nz

<https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html> states:
"The total number of input + output + goto operands is limited to 30."

It appears a "+" input operand is internally counted as a dual input plus
output operand which limits the total number of such registers to 15:

30_operands.c:
#include <stdint.h>

typedef uint8_t u8x16_t __attribute__ ((vector_size (16)));

int main(void) {
  register uint64_t rdi asm ("rdi");
  register uint64_t rsi asm ("rsi");
  register uint64_t r10 asm ("r10");
  register uint64_t r11 asm ("r11");
  register uint64_t rbx asm ("rbx");
  register uint64_t rbp asm ("rbp");
  register uint64_t r12 asm ("r12");
  register uint64_t r14 asm ("r14");
  register uint64_t r15 asm ("r15");
  register uint64_t r13 asm ("r13");
  register u8x16_t xmm2 asm ("xmm2");
  register u8x16_t xmm3 asm ("xmm3");
  register u8x16_t xmm4 asm ("xmm4");
  register u8x16_t xmm5 asm ("xmm5");
  register u8x16_t xmm6 asm ("xmm6");
  register u8x16_t xmm7 asm ("xmm7");

  //15 operands is OK
  asm volatile ("" : "+r" (rdi),   "+r" (rsi),   "+r" (r10),   "+r" (r11),  
"+r" (rbx),   "+r" (rbp),
             "+r" (r12),   "+r" (r14),   "+r" (r15),   "+r" (r13),   "+x"
(xmm2),  "+x" (xmm3),
             "+x" (xmm4),  "+x" (xmm5),  "+x" (xmm6));

  //16 operands
  asm volatile ("" : "+r" (rdi),   "+r" (rsi),   "+r" (r10),   "+r" (r11),  
"+r" (rbx),   "+r" (rbp),
             "+r" (r12),   "+r" (r14),   "+r" (r15),   "+r" (r13),   "+x"
(xmm2),  "+x" (xmm3),
             "+x" (xmm4),  "+x" (xmm5),  "+x" (xmm6),  "+x" (xmm7));

  register u8x16_t xmm8 asm ("xmm8");
  register u8x16_t xmm9 asm ("xmm9");
  register u8x16_t xmm10 asm ("xmm10");
  register u8x16_t xmm11 asm ("xmm11");
  register u8x16_t xmm12 asm ("xmm12");
  register u8x16_t xmm13 asm ("xmm13");
  register u8x16_t xmm14 asm ("xmm14");
  register u8x16_t xmm15 asm ("xmm15");

  //24 operands
  asm volatile ("" : "+r" (rdi),   "+r" (rsi),   "+r" (r10),   "+r" (r11),  
"+r" (rbx),   "+r" (rbp),
             "+r" (r12),   "+r" (r14),   "+r" (r15),   "+r" (r13),   "+x"
(xmm2),  "+x" (xmm3),
             "+x" (xmm4),  "+x" (xmm5),  "+x" (xmm6),  "+x" (xmm7),  "+x"
(xmm8),  "+x" (xmm9),
             "+x" (xmm10), "+x" (xmm11), "+x" (xmm12), "+x" (xmm13), "+x"
(xmm14), "+x" (xmm15));

  register u8x16_t xmm0 asm ("xmm0");
  register u8x16_t xmm1 asm ("xmm1");
  register uint64_t rax asm ("rax");
  register uint64_t rcx asm ("rcx");
  register uint64_t rdx asm ("rdx");
  register uint64_t r8 asm ("r8");
  register uint64_t r9 asm ("r9");

  //31 operands
  asm volatile ("" : "+r" (rdi),   "+r" (rsi),   "+r" (r10),   "+r" (r11),  
"+r" (rbx),   "+r" (rbp),
             "+r" (r12),   "+r" (r14),   "+r" (r15),   "+r" (r13),   "+x"
(xmm2),  "+x" (xmm3),
             "+x" (xmm4),  "+x" (xmm5),  "+x" (xmm6),  "+x" (xmm7),  "+x"
(xmm8),  "+x" (xmm9),
             "+x" (xmm10), "+x" (xmm11), "+x" (xmm12), "+x" (xmm13), "+x"
(xmm14), "+x" (xmm15),
             "+x" (xmm0),  "+x" (xmm1),  "+r" (rax),   "+r" (rcx),   "+r"
(rdx),   "+r" (r8),
             "+r" (r9));

  register uint64_t mmx0 asm ("mm0");
  register uint64_t mmx1 asm ("mm1");
  register uint64_t mmx2 asm ("mm2");
  register uint64_t mmx3 asm ("mm3");
  register uint64_t mmx4 asm ("mm4");
  register uint64_t mmx5 asm ("mm5");
  register uint64_t mmx6 asm ("mm6");
  register uint64_t mmx7 asm ("mm7");

  //39 operands
  asm volatile ("" : "+r" (rdi),   "+r" (rsi),   "+r" (r10),   "+r" (r11),  
"+r" (rbx),   "+r" (rbp),
             "+r" (r12),   "+r" (r14),   "+r" (r15),   "+r" (r13),   "+x"
(xmm2),  "+x" (xmm3),
             "+x" (xmm4),  "+x" (xmm5),  "+x" (xmm6),  "+x" (xmm7),  "+x"
(xmm8),  "+x" (xmm9),
             "+x" (xmm10), "+x" (xmm11), "+x" (xmm12), "+x" (xmm13), "+x"
(xmm14), "+x" (xmm15),
             "+x" (xmm0),  "+x" (xmm1),  "+r" (rax),   "+r" (rcx),   "+r"
(rdx),   "+r" (r8),
             "+r" (r9),    "+y" (mmx0),  "+y" (mmx1),  "+y" (mmx2),  "+y"
(mmx3),  "+y" (mmx4),
             "+y" (mmx5),  "+y" (mmx6),  "+y" (mmx7));

  return 0;
}

$ gcc-snapshot.sh -O3 30_operands.c
30_operands.c: In function 'main':
30_operands.c:29:3: error: more than 30 operands in 'asm'
   asm volatile ("" : "+r" (rdi),   "+r" (rsi),   "+r" (r10),   "+r" (r11),  
"+r" (rbx),   "+r" (rbp),
   ^
30_operands.c:43:3: error: more than 30 operands in 'asm'
   asm volatile ("" : "+r" (rdi),   "+r" (rsi),   "+r" (r10),   "+r" (r11),  
"+r" (rbx),   "+r" (rbp),
   ^
30_operands.c:57:3: error: more than 30 operands in 'asm'
   asm volatile ("" : "+r" (rdi),   "+r" (rsi),   "+r" (r10),   "+r" (r11),  
"+r" (rbx),   "+r" (rbp),
   ^
30_operands.c:74:3: error: more than 30 operands in 'asm'
   asm volatile ("" : "+r" (rdi),   "+r" (rsi),   "+r" (r10),   "+r" (r11),  
"+r" (rbx),   "+r" (rbp),
   ^

CLANG compiles the program:
$ clang-3.6 -Wall -O3 30_operands.c
$

Due to the double counting of +input operands it would be sensible for the
operand limit to be at least twice the number of architecture registers. For
x64 the minimum should be 2*(16 GPR + 16 XMM + 8 MMX) = 80 operands. This
minimum does not include provision for goto operands.