[Bug target/62166] New: Poor code generation (x86-64)

[adam at consulting dot net.nz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62166

            Bug ID: 62166
           Summary: Poor code generation (x86-64)
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: adam at consulting dot net.nz

$ gcc-snapshot.sh --version
gcc (Debian 20140814-1) 4.10.0 20140814 (experimental) [trunk revision 213954]

weird_code_gen.c:

#include <stdint.h>

typedef void (*f_t)(uint64_t rdi, uint64_t rsi, uint64_t rdx, uint64_t rcx);
void weird_code_gen(uint64_t rdi, uint64_t rsi, uint64_t rdx, uint64_t rcx);
f_t dispatch[] = {&weird_code_gen};

void weird_code_gen(uint64_t rdi, uint64_t rsi, uint64_t rdx, uint64_t rcx) {
  int64_t s8 = (int8_t) rcx;
  uint32_t u8 = (uint8_t) (rcx >> 8);
  //asm volatile ("" : "+r" (rcx));
  rdx += s8;
  rcx >>= 16;
  dispatch[u8](rdi, rsi, rdx, rcx);
}

int main(void) {
  return 0;
}

$ gcc-snapshot.sh -O3 weird_code_gen.c && objdump -d -m i386:x86-64:intel a.out
|less

00000000004004c0 <weird_code_gen>:
  4004c0:       48 89 c8                mov    rax,rcx
  4004c3:       53                      push   rbx
  4004c4:       0f b6 dd                movzx  ebx,ch
  4004c7:       48 0f be c0             movsx  rax,al
  4004cb:       48 c1 e9 10             shr    rcx,0x10
  4004cf:       48 01 c2                add    rdx,rax
  4004d2:       48 8b 04 dd d8 08 60    mov    rax,QWORD PTR [rbx*8+0x6008d8]
  4004d9:       00 
  4004da:       5b                      pop    rbx
  4004db:       ff e0                   jmp    rax


Code generation with asm volatile uncommented:

00000000004004c0 <weird_code_gen>:
  4004c0:       4c 0f be c1             movsx  r8,cl
  4004c4:       0f b6 c5                movzx  eax,ch
  4004c7:       89 c0                   mov    eax,eax
  4004c9:       48 c1 e9 10             shr    rcx,0x10
  4004cd:       4c 01 c2                add    rdx,r8
  4004d0:       ff 24 c5 d0 08 60 00    jmp    QWORD PTR [rax*8+0x6008d0]

The asm volatile workaround fails if u8 is of type uint64_t.