Optimization bug in Apple's LLVM, or bug in code? - ios

Question

I have some iOS C++ code that compiles correctly on my local machine (LLVM 9.0) but compiles incorrectly on my build server (LLVM 10.0). The project is generated via CMake (same version on both) so the code being compiled is the same, with the same compiler settings.
After finally realizing that some critical values weren't being updated on the LLVM10 version I investigated the assembly and found out it was completely skipping part of the code.
void SceneDisplay::SetSize(const math::Vec2 &Size)
{
m_Size = Size;
m_ScreenWidth = int(m_Size.x * float(GraphicsUtil::WIDTH));
m_ScreenHeight = int(m_Size.y * float(GraphicsUtil::HEIGHT));
UpdateOffsetScale();
}
m_Size is initialized to 1.0,1.0 in the class constructor. This works fine and everything is perfect with LLVM9 - with LLVM10 we get the following disassembly:
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset %rbp, -16
movq %rsp, %rbp
.cfi_def_cfa_register %rbp
subq $16, %rsp
movq __ZN12GraphicsUtil6HEIGHTE#GOTPCREL(%rip), %rax
movq __ZN12GraphicsUtil5WIDTHE#GOTPCREL(%rip), %rcx
movq %rdi, -8(%rbp)
movq %rsi, -16(%rbp)
movq -8(%rbp), %rsi
Ltmp2347:
movq -16(%rbp), %rdi
movq (%rdi), %rdi
movq %rdi, 56(%rsi)
movl (%rcx), %edx
movl %edx, 12(%rsi)
movl (%rax), %edx
movl %edx, 16(%rsi)
movq (%rsi), %rax
movq %rsi, %rdi
callq *136(%rax)
addq $16, %rsp
popq %rbp
retq
As you can see the assignment of the two member variables is completely 'optimized' to just assume that m_Size.x and m_Size.y are 1.0 - thus just copying the values of GraphicsUtil::WIDTH and HEIGHT.
I fixed this by changing the code to use "Size" instead of "m_Size" for those assignments, as well as making them volatile just in case. But I'm wondering if there is a legitimate compiler error here or I'm missing something?
Edit: It should be noted that m_Size is nearly never 1.0,1.0
Edit2: The correct assembly for the assignments, as generated on my machine (different arch though, not able to get the same arch as above right now)
str x8, [x0, #56]
lsr x9, x8, #32
fmov s0, w8
adrp x8, __ZN12GraphicsUtil5WIDTHE#GOTPAGE
ldr x8, [x8, __ZN12GraphicsUtil5WIDTHE#GOTPAGEOFF]
ldr s1, [x8]
ucvtf s1, s1
fmul s0, s0, s1
fcvtzs w8, s0
str w8, [x0, #12]
fmov s0, w9
adrp x8, __ZN12GraphicsUtil6HEIGHTE#GOTPAGE
ldr x8, [x8, __ZN12GraphicsUtil6HEIGHTE#GOTPAGEOFF]
ldr s1, [x8]
ucvtf s1, s1
fmul s0, s0, s1
fcvtzs w8, s0
str w8, [x0, #16]

Answer 1

After making a minimal test case I was able to confirm it's definitely a compiler bug.
Conditions: No other piece of code modifies m_Size, m_Size is initialized math::Vec2 m_Size{1.0, 1.0};. It works perfectly on every version of LLVM I could find before 10.0, seems some sort of regression occurred at that version.
Have submitted to Apple's LLVM team and llvm.org.
Thanks for comments.