[LLVMbugs] [Bug 19612] New: bad codegen on MIPS va_arg

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=19612

            Bug ID: 19612
           Summary: bad codegen on MIPS va_arg
           Product: clang
           Version: trunk
          Hardware: Other
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: msebor at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Compiling the following function for MIPS N32 or N64 with clang 3.4 or the
latest trunk results in incorrect object code that ends up extracting zeros
instead of the values passed in through the variable argument list.

int foo (int a, ...)
{
    __builtin_va_list va;

    __builtin_va_start (va, a);

    int n = __builtin_va_arg (va, int);

    __builtin_va_end (va);

    return n;
}

In the generated object code, note the sd instruction at foo+1c which stores a
double word at the destination address on the stack, and the lw instruction at
foo+30 which reads a word from the same address. Since only the high word of
the stored double word is read, the low half which contains the value of the
argument is discarded.

00000000 <foo>:
   0:    27bdffc0     addiu    sp,sp,-64
   4:    ffab0038     sd    a7,56(sp)
   8:    ffaa0030     sd    a6,48(sp)
   c:    ffa90028     sd    a5,40(sp)
  10:    ffa80020     sd    a4,32(sp)
  14:    ffa70018     sd    a3,24(sp)
  18:    ffa60010     sd    a2,16(sp)
  1c:    ffa50008     sd    a1,8(sp)   ; note sd
  20:    27a20008     addiu    v0,sp,8
  24:    afa20004     sw    v0,4(sp)
  28:    24420008     addiu    v0,v0,8
  2c:    afa20004     sw    v0,4(sp)
  30:    8fa20008     lw    v0,8(sp)   ; note lw 
  34:    03e00008     jr    ra
  38:    27bd0040     addiu    sp,sp,64

0000003c <$tmp1>:
  3c:    00000000     nop

The following options were used to compile the function:

clang -O2 --target=mips64-unknown-linux-gnu -c -mabi=n32 t.c

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