(inline assembly in C) Assembler messages: Error: unknown pseudo-op:

Question

I have written a short C "wrapper" function for an asm inline assembly, as below. The assembly code consists of a while loop, computing several vector dot product using SSE2. I am using GCC 4.8.4 on Ubuntu 14.04 on an x86. The following code can be assembled "without problem" under

gcc -fpic -O2 -msse2 -S foo.c

But when I do

gcc -c foo.s

an error is triggered:

foo.c: Assembler messages:
foo.c:2: Error: unknown pseudo-op: `.while5'

I checked the assembler ouput "foo.s" and found something strange.

C file "foo.c":

#include <emmintrin.h>

void foo (int kk, double *A, double *B, double *ALPHA, double *C, int ldc) {
   asm("movl %0, %%ecx\n\t"  /* kk -> %ecx */
       "movl %3, %%eax\n\t"  /* A -> %eax */
       "movl %4, %%edx\n\t"  /* B -> %edx */
       /* a while-loop */
       ".while%=\n\t"
       "movsd   (%%edx), %%xmm5\n\t"
       "unpcklpd %%xmm5, %%xmm5\n\t"
       "movapd  %%xmm5, %%xmm6\n\t"
       "movapd  (%%eax), %%xmm4\n\t"
       "mulpd   %%xmm4, %%xmm6\n\t"
       "movapd  16(%%eax), %%xmm7\n\t"
       "addl    $32, %%eax\n\t"
       "addpd   %%xmm6, %%xmm0\n\t"
       "mulpd   %%xmm7, %%xmm5\n\t"
       "addpd   %%xmm5, %%xmm1\n\t"
       "movsd   8(%%edx), %%xmm6\n\t"
       "addl    $16, %%edx\n\t"
       "unpcklpd %%xmm6, %%xmm6\n\t"
       "mulpd   %%xmm6, %%xmm4\n\t"
       "addpd   %%xmm4, %%xmm2\n\t"
       "mulpd   %%xmm6, %%xmm7\n\t"
       "addpd   %%xmm7, %%xmm3\n\t"
       "subl    $1, %%ecx\n\t"  /* kk-- */
       "testl   %%ecx, %%ecx\n\t"  /* kk = 0 ? */
       "jne .while%=\n\t"
        /* other input operands passing */
       "movl %5, %%ecx\n\t"  /* C -> %ecx */
       "movl %1, %%eax\n\t"  /* ALPHA -> %eax, then C0 -> %eax */
       "movl %2, %%edx\n\t"  /* ldc -> %edx */
       /* write-back */
       "movsd (%%eax), %%xmm7\n\t"
       "unpcklpd %%xmm7, %%xmm7\n\t"
       "leal (%%ecx,%%edx,8), %%eax\n\t"  /* C0=C+ldc */
       "mulpd %%xmm7, %%xmm0\n\t"
       "addpd (%%ecx), %%xmm0\n\t"
       "movapd %%xmm0, (%%ecx)\n\t"
       "mulpd %%xmm7, %%xmm2\n\t"
       "addpd (%%eax), %%xmm2\n\t"
       "movapd %%xmm2, (%%eax)\n\t"
       "mulpd %%xmm7, %%xmm1\n\t"
       "addpd 16(%%ecx), %%xmm1\n\t"
       "movapd %%xmm1, 16(%%ecx)\n\t"
       "mulpd %%xmm7, %%xmm3\n\t"
       "addpd 16(%%eax), %%xmm3\n\t"
       "movapd %%xmm3, 16(%%eax)\n\t"
       : /* no output operands */
       : "m"(kk), "m"(ALPHA), "m"(ldc), "m"(A), "m"(B), "m"(C)  /* input operands */
       : "eax", "edx", "ecx", "xmm0", "xmm1", "xmm2", "xmm3", "xmm4", "xmm5", "xmm6", "xmm7"  /* clobbers */ );
   }

assembler output (the while-loop looks odd!)

.LFB503:
            .cfi_startproc
 #APP
 # 4 "foo.c" 1
            movl 4(%esp), %ecx
            movl 8(%esp), %eax
            movl 12(%esp), %edx
            .while5
            movsd   (%edx), %xmm5
            unpcklpd %xmm5, %xmm5
            movapd  %xmm5, %xmm6
            movapd  (%eax), %xmm4
            mulpd   %xmm4, %xmm6
            movapd  16(%eax), %xmm7
            addl    $32, %eax
            addpd   %xmm6, %xmm0
            mulpd   %xmm7, %xmm5
            addpd   %xmm5, %xmm1
            movsd   8(%edx), %xmm6
            addl    $16, %edx
            unpcklpd %xmm6, %xmm6
            mulpd   %xmm6, %xmm4
            addpd   %xmm4, %xmm2
            mulpd   %xmm6, %xmm7
            addpd   %xmm7, %xmm3
            subl    $1, %ecx
            testl   %ecx, %ecx
            jne .while5
            movl 20(%esp), %ecx
            movl 16(%esp), %eax
            movl 24(%esp), %edx
            movsd (%eax), %xmm7
            unpcklpd %xmm7, %xmm7
            leal (%ecx,%edx,8), %eax
            mulpd %xmm7, %xmm0
            addpd (%ecx), %xmm0
            movapd %xmm0, (%ecx)
            mulpd %xmm7, %xmm2
            addpd (%eax), %xmm2
            movapd %xmm2, (%eax)
            mulpd %xmm7, %xmm1
            addpd 16(%ecx), %xmm1
            movapd %xmm1, 16(%ecx)
            mulpd %xmm7, %xmm3
            addpd 16(%eax), %xmm3
            movapd %xmm3, 16(%eax)      
# 0 "" 2
#NO_APP
            ret
            .cfi_endproc

Can anyone kindly refer to me what has happened? I don't think it is my compiler's problem. There must be something wrong with my code. Thx!

You are missing a colon to make it a label: .while%=:. Note that -S does not assemble, it just produces assembly output so it can contain errors too. — Jester
@AlphaBetaGamma Ah! That's supposed to be a label. Typically, you don't indent labels and if you make a local label on x86, it should begin with .L, so .Lwhile:. I thought that was an attempt to use a nonexisting pseudo-opcode named .while. — fuz
Any time you're writing mov instructions in inline asm, you should look for a way to let the compiler do it, so it can avoid unnecessary instructions in some cases. e.g. for x86-64, the args will already all be in integer registers, so it's silly to do a bunch of mov reg,reg instead of just letting the compiler choose regs for you. (see the x86 tag wiki for a link to my inline asm examples of how to let the compiler do as much as possible.) — Peter Cordes

Craig Estey Craig Estey · Accepted Answer · 2016-02-03T23:46:15

Because your .while is not defined as a label, it's being seen as a [non-existent] pseudo-op.

Change:

".while%=\n\t"

Into:

".while%=:\n\t"

UPDATE:

Per your request.

A "pseudo-op" is [terminology for] an assember directive that doesn't correspond to an instruction.

Some examples:

.globl main to specify that the main label is a global variable.

.text to specify that what follows should be placed in the "text" segment (likewise for .data).

The . prefix is [generally] reserved for pseudo ops. That's why you got the Error: unknown pseudo-op: message.

If you had done "while%=\n\t" instead [still wrong because there was no : to denote a label], you would have gotten a different message: Error: no such instruction:

(inline assembly in C) Assembler messages: Error: unknown pseudo-op:

1 Answers