Call to custom cotd function taking argument by reference seemingly elided

Ryan_L_1 · ‎06-23-2016

I've got some code which contains a function with signature

/* cotd.h */
#ifndef COTD_PTR_H
#define COTD_PTR_H

void cotd(double *x);

#endif

that updates the value in x, say by adding 2 for demonstration. When I invoke cotd with the address of a double, the value is never changed, and the assembly from passing /S contains no call to cotd when cotd is in a separate compilation unit from the call and no debug flags (e.g. /Z7) are passed.

Details follow.

Implementation:

/* cotd.c */
#include "cotd.h"
void cotd(double *x) {
    *x = 2.0+*x;
}

Useage:

/* cotdissue.c */
#include "stdio.h"
#include "cotd.h"

int main(void) {
    double x = 1.23;
    printf("Initial x = %g\n", x);
    cotd(&x);
    printf("x = %g; expected = 3.23\n",x);
    return 0;
}

Compiling without /Z7 and executing produces:

icl  /c /nologo /MD /I "Z:\sandbox\rlivings\temp\sparse\iclrepro"  "cotdissue.c" 
icl  /c /nologo /MD /I "Z:\sandbox\rlivings\temp\sparse\iclrepro"  "cotd.c" 

link  /nologo /NODEFAULTLIB:LIBCMT  /OUT:cotdissue.exe   cotdissue.obj cotd.obj   
Build completed using compiler intelc13msvs2013 

cotdissue.exe
Initial x = 1.23 
x = 1.23; expected = 3.23

showing that x was never updated on the call to cotd. The assembly generated from cotdissue.c is:

main	PROC 
.B1.1::                         ; Preds .B1.0
        sub       rsp, 40                                       ;4.16
        mov       rdx, 0000000000H                              ;4.16
        mov       ecx, 3                                        ;4.16
        call      __intel_new_feature_proc_init                 ;4.16
                                ; LOE rbx rbp rsi rdi r12 r13 r14 r15 xmm6 xmm7 xmm8 xmm9 xmm10 xmm11 xmm12 xmm13 xmm14 xmm15
.B1.6::                         ; Preds .B1.1
        stmxcsr   DWORD PTR [32+rsp]                            ;4.16
        lea       rcx, QWORD PTR [_2__STRING.0.0.1]             ;6.5
        or        DWORD PTR [32+rsp], 32832                     ;4.16
        ldmxcsr   DWORD PTR [32+rsp]                            ;4.16
        movsd     xmm1, QWORD PTR [_2il0floatpacket.0]          ;6.5
        movd      rdx, xmm1                                     ;6.5
        call      QWORD PTR [__imp_printf]                      ;6.5
                                ; LOE rbx rbp rsi rdi r12 r13 r14 r15 xmm6 xmm7 xmm8 xmm9 xmm10 xmm11 xmm12 xmm13 xmm14 xmm15
.B1.2::                         ; Preds .B1.6
        movsd     xmm1, QWORD PTR [_2il0floatpacket.0]          ;8.5
        lea       rcx, QWORD PTR [_2__STRING.1.0.1]             ;8.5
        movd      rdx, xmm1                                     ;8.5
        call      QWORD PTR [__imp_printf]                      ;8.5
                                ; LOE rbx rbp rsi rdi r12 r13 r14 r15 xmm6 xmm7 xmm8 xmm9 xmm10 xmm11 xmm12 xmm13 xmm14 xmm15
.B1.3::                         ; Preds .B1.2
        xor       eax, eax                                      ;9.12
        add       rsp, 40                                       ;9.12
        ret                                                     ;9.12
        ALIGN     16
                                ; LOE
.B1.4::
; mark_end;
main ENDP

Some observations:

Passing /Z7 produces the desired result, 3.23 for x
There are no warnings that cotd may conflict with a built-in function. Are we possibly getting side effect information from the known trig function?
Changing the name of cotd to bcotd produces expected results without /Z7

Any thoughts would be appreciated. I know that I can rename my function as a workaround but the incorrect answer without any notification is concerning to me.

Anoop_M_Intel · ‎06-23-2016

Hi Ryan,

I can reproduce this problem on my end. I can check with the compiler development engineers on this. In the meantime, you can try the following:

>icl cotdissue.c cotd.c /Qno-builtin-cotd
Intel(R) C++ Intel(R) 64 Compiler for applications running on Intel(R) 64, Version 16.0.2.180 Build 20160204
Copyright (C) 1985-2016 Intel Corporation. All rights reserved.

-out:cotdissue.exe
cotdissue.obj
cotd.obj

>cotdissue.exe
Initial x = 1.23
x = 3.23; expected = 3.23

The compiler option /Qno-builtin-<function name> will not link to the builtin function rather link with the user defined one as you can above. Documentation on this option is available at https://software.intel.com/en-us/node/581706.

Thanks and Regards
Anoop

Ryan_L_1 · ‎06-23-2016

Thanks for the quick reply Anoop. I confirm that your workaround succeeds and am interested to hear what the consensus on the behavior is.