Is it now wrong to "return" from a "switch" statement in C++11? [duplicate]

Question

Using VS2012, I noticed that a switch that's been working for several years now seems to be broken in Release builds but works correctly (or at least as it used to) in Debug builds. I can't see anything at all wrong with the code so would appreciate some feedback on the correctness of using return statements from within a switch block.

The following code compiles ok but gives the wrong output in a Release build on Win7 32-bit...

#include <stdio.h> #include <tchar.h>  class CSomeClass { public:     float GetFloat(int nInt)     {         printf("GetFloat() - entered\n");         switch (nInt)         {         case 1 :             printf("GetFloat() - case 1 entered\n");             return 0.5F;         case 0 :             printf("GetFloat() - case 0 entered\n");             return 1.0F;         case 2 :             printf("GetFloat() - case 2 entered\n");             return 2.0F;         case 3 :             printf("GetFloat() - case 3 entered\n");             return 3.0F;         case 4 :             printf("GetFloat() - case 4 entered\n");             return 4.0F;         }         printf("GetFloat() - exit\n");         return 1.0F;     } };  int _tmain(int argc, _TCHAR* argv[]) {     CSomeClass pClass;     float fValue = pClass.GetFloat(3);     printf("fValue = %f\n", fValue);      return 0; } 

If you can repeat the problem, and have a MS Connect login, maybe you can vote it up here too?

Actual results

Release build gives the following incorrect result:

GetFloat() - entered GetFloat() - case 3 entered fValue = 0.000000 

Expected results

Debug build gives the following correct result:

GetFloat() - entered GetFloat() - case 3 entered fValue = 3.000000 

MS Connect bug report

Answer 1

It sounds like it might be this bug? Where there is a problem in returning floating point values generated in a similar way?

Answer 2

Definitely a compiler error. Here is the stripped down asm code being executed (jumps etc. removed). The compiler removes some code it assumes to be unnecessary - even though it is not.

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Release build:

// inside GetFloat 00E0104D  fld         dword ptr ds:[0E021D8h]  // load constant float onto FPU stack 00E01068  add         esp,4 00E0106B  ret  // back in main 00E01098  cvtss2sd    xmm0,xmm0  // convert float to double. assumes the returned value to be in xmm0 00E0109C  sub         esp,8   00E0109F  movsd       mmword ptr [esp],xmm0  // push double being printed (xmm0 is with high likelyhood = 0) 00E010A4  push        0E021B8h   // push output string 00E010A9  call        dword ptr ds:[0E02090h]  // call printf 

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Debug build:

003314B0  fld         dword ptr ds:[335964h]   // load const float onto FPU stack [...] 00331500  mov         esp,ebp   00331502  pop         ebp   00331503  ret         4   // back in main 00331598  fstp        dword ptr [fValue]  // copies topmost element of the FPU stack to [fValue] 0033159B  cvtss2sd    xmm0,dword ptr [fValue] // correctly takes returned value (now inside [fValue] for conversion to double  003315A0  mov         esi,esp   003315A2  sub         esp,8   003315A5  movsd       mmword ptr [esp],xmm0  // push double being printed 003315AA  push        335940h  // push output string 003315AF  call        dword ptr ds:[3392C8h]  // call printf