Microsoft VS 2015 compiler bug? - c++

Question

I have the following function:
void MainLayer::onMouseScroll(cocos2d::EventMouse* event)
{
const float scrollAmount = event->getScrollY();
const float newMapScale = scrollAmount < 0.f ? std::min(_mapScale * k_deltaMapScaleMag, k_maxMapScale) : std::max(_mapScale * k_deltaMapScaleMin, k_minMapScale);
const float invNewMapScale = 1.f / newMapScale;
const Vec2 anchorScreenSpace = event->getLocationInView();
_mapScale = newMapScale;
_invMapScale = invNewMapScale;
updateMapTransform();
}
Microsoft VS 2015 compiler generates the following assembly:
push ebp
mov ebp, esp
push -1
push __ehhandler$?onMouseScroll#MainLayer##AAEXPAVEventMouse#cocos2d###Z
mov eax, DWORD PTR fs:0
push eax
sub esp, 20 ; 00000014H
mov eax, DWORD PTR ___security_cookie
xor eax, ebp
mov DWORD PTR __$ArrayPad$[ebp], eax
push esi
push eax
lea eax, DWORD PTR __$EHRec$[ebp]
mov DWORD PTR fs:0, eax
mov esi, ecx
mov ecx, DWORD PTR _event$[ebp]
; File c:\program files (x86)\microsoft visual studio 14.0\vc\include\algorithm
; 3649 : return (_DEBUG_LT(_Right, _Left) ? _Right : _Left);
lea edx, DWORD PTR $T1[ebp]
; File e:\projects\aliceandbob\classes\mainlayer.cpp
; 121 : const float newMapScale = scrollAmount < 0.f ? std::min(_mapScale * k_deltaMapScaleMag, k_maxMapScale) : std::max(_mapScale * k_deltaMapScaleMin, k_minMapScale);
movss xmm1, DWORD PTR __real#3f800000
xorps xmm0, xmm0
comiss xmm0, DWORD PTR [ecx+56]
movss xmm0, DWORD PTR [esi+712]
jbe SHORT $LN3#onMouseScr
mulss xmm0, DWORD PTR __real#3f99999a
; File c:\program files (x86)\microsoft visual studio 14.0\vc\include\algorithm
; 3649 : return (_DEBUG_LT(_Right, _Left) ? _Right : _Left);
mov eax, OFFSET ?k_maxMapScale#?A0x69cbde3b##3MB
comiss xmm0, DWORD PTR __real#41200000
; File e:\projects\aliceandbob\classes\mainlayer.cpp
; 121 : const float newMapScale = scrollAmount < 0.f ? std::min(_mapScale * k_deltaMapScaleMag, k_maxMapScale) : std::max(_mapScale * k_deltaMapScaleMin, k_minMapScale);
jmp SHORT $LN18#onMouseScr
$LN3#onMouseScr:
mulss xmm0, DWORD PTR __real#3f4ccccd
; File c:\program files (x86)\microsoft visual studio 14.0\vc\include\algorithm
; 3612 : return (_DEBUG_LT(_Left, _Right) ? _Right : _Left);
mov eax, OFFSET ?k_minMapScale#?A0x69cbde3b##3MB
comiss xmm1, xmm0
$LN18#onMouseScr:
cmovbe eax, edx
movss xmm0, DWORD PTR [eax]
; File e:\projects\aliceandbob\classes\mainlayer.cpp
; 124 : const Vec2 anchorScreenSpace = event->getLocationInView();
lea eax, DWORD PTR _anchorScreenSpace$[ebp]
divss xmm1, xmm0
push eax
movss DWORD PTR tv161[ebp], xmm0
movss DWORD PTR _invNewMapScale$1$[ebp], xmm1
call DWORD PTR __imp_?getLocationInView#EventMouse#cocos2d##QBE?AVVec2#2#XZ
; 125 : _mapScale = newMapScale;
movss xmm0, DWORD PTR tv161[ebp]
; 126 : _invMapScale = invNewMapScale;
; 127 :
; 128 : updateMapTransform();
mov ecx, esi
mov DWORD PTR __$EHRec$[ebp+8], 0
movss DWORD PTR [esi+712], xmm0
movss xmm0, DWORD PTR _invNewMapScale$1$[ebp]
movss DWORD PTR [esi+716], xmm0
call ?updateMapTransform#MainLayer##AAEXXZ ; MainLayer::updateMapTransform
; 129 : }
mov ecx, DWORD PTR __$EHRec$[ebp]
mov DWORD PTR fs:0, ecx
pop ecx
pop esi
mov ecx, DWORD PTR __$ArrayPad$[ebp]
xor ecx, ebp
call #__security_check_cookie#4
mov esp, ebp
pop ebp
ret 4
The problem is the compiler generates the code that does not store the result of the expression into newMapScale variable. As you see, it also does not insert minss and maxss instructions here (for std::max() and std::min()). If I deny compuler to use SSE instructions - it works as intended. I tried to reproduce this issue on a test project and failed - almost exact same code had minss and maxss instructions and stored the result of expression into the newMapScale variable. Do I have some kind of undefined behavior, or it's just a compiler bug?
I use VS 2015 Update 3, VS 2015 - Windows XP (v140_xp) SDK.
UPD: I only left source code that helps reproducing the problem. Removing any line from that code fixes compiler behavior. Please pay attention to the following lines:
movss xmm1, DWORD PTR __real#3f800000
xorps xmm0, xmm0
comiss xmm0, DWORD PTR [ecx+56]
movss xmm0, DWORD PTR [esi+712]
jbe SHORT $LN3#onMouseScr
mulss xmm0, DWORD PTR __real#3f99999a
; File c:\program files (x86)\microsoft visual studio 14.0\vc\include\algorithm
; 3649 : return (_DEBUG_LT(_Right, _Left) ? _Right : _Left);
mov eax, OFFSET ?k_maxMapScale#?A0x69cbde3b##3MB
comiss xmm0, DWORD PTR __real#41200000
; File e:\projects\aliceandbob\classes\mainlayer.cpp
; 121 : const float newMapScale = scrollAmount < 0.f ? std::min(_mapScale * k_deltaMapScaleMag, k_maxMapScale) : std::max(_mapScale * k_deltaMapScaleMin, k_minMapScale);
jmp SHORT $LN18#onMouseScr
$LN3#onMouseScr:
mulss xmm0, DWORD PTR __real#3f4ccccd
; File c:\program files (x86)\microsoft visual studio 14.0\vc\include\algorithm
; 3612 : return (_DEBUG_LT(_Left, _Right) ? _Right : _Left);
mov eax, OFFSET ?k_minMapScale#?A0x69cbde3b##3MB
comiss xmm1, xmm0
$LN18#onMouseScr:
cmovbe eax, edx
movss xmm0, DWORD PTR [eax]
; File e:\projects\aliceandbob\classes\mainlayer.cpp
; 124 : const Vec2 anchorScreenSpace = event->getLocationInView();
lea eax, DWORD PTR _anchorScreenSpace$[ebp]
divss xmm1, xmm0
push eax
As Andrey Turkin said, xmm0 gets discarded although it contains calculation results. An unitialized memory is loaded into xmm0 instead.