Some specially crafted shaders exhibit unexpected branching behavior

Description
I'm observing some odd behavior in some shaders resulting in incorrect branching. The issues occurs in debug with -Od. The behavior seems triggered by certain sequences of operations involving texture loads and subsequent dependent branches. With -Od, adding code that shouldn't have any changes or side-effects in execution can affect the result of the branches.

Steps to Reproduce

Compile the following pixel shader with -T ps_6_6 -E PSMain -Od (Compiler Explorer: https://godbolt.org/z/nKx5qj33s). I was able reproduce this using the DirectX-Graphics-Samples hello triangle example.

#define TOGGLE_BUG 1
struct PSInput
{
  float4 position :SV_POSITION;
  float2 uv :TEXCOORD;
};

Texture2D<uint4> g_texture : register(t0);

float3 bugtest()
{
  float should_be_zerof = 0.0f;
  float4 should_be_onef4 = 1;

  uint test_flags = (1 << 0);
  bool test_false = (test_flags & (1 << 3)) ? 1 : 0;
  bool test_true = (test_flags & (1 << 0)) ? 1 : 0;
  float close_to_zerof = 1e-20;

  uint one_or_zero = (g_texture[uint2(0, 0)].w & 0x1);
  uint uint_zero = uint(one_or_zero * close_to_zerof);
  float zerof = 0.0f + close_to_zerof * (uint_zero != 0 ? 1.0f : 0.0f);

  float onef = 1.0;
  float4 zerof4 = 0.0f;

#if TOGGLE_BUG
  if (test_false)
  {
    zerof4.x = 0.0f;
  }
  else
  {
    zerof4.x = 0.0f;
  }
#endif

  onef = saturate(1.0f + close_to_zerof * zerof);
  should_be_onef4 = saturate(1.0f + close_to_zerof * zerof4);
  if (test_true)
  {
    should_be_zerof = saturate(1.0f - onef);
  }

  float3 red = saturate(should_be_onef4.xyz * close_to_zerof + float3(1.0, 0.0f, 0));
  float3 green = saturate(should_be_zerof * close_to_zerof + float3(0.0f, 1.0f, 0));

  if (should_be_zerof < 1.0f) 
  {
    // this should always be true
    return green;
  }
  else
  {
    return red; // wat?
  }
}

float4 PSMain(PSInput input) : SV_TARGET
{
  float3 out_color = bugtest();
  return float4(out_color, 1.0);
}

Actual Behavior
This code should always evaluate to producing a green pixel but ends up taking a wrong branch and produces red pixels. Changing the TOGGLE_BUG to 0 removes some extra code that shouldn't have side effects but changes the behavior with -Od and this results in the correct branch being taken at the end of the shader.

Environment

• DXC version 1.8.2502
• Host Operating System Windows 11 23H2

Compile the supplied PSMain shader with DXC 1.8.2502 using -T ps_6_6 -E PSMain -Od, then compare behavior with TOGGLE_BUG set to 1 and 0. Trace the compiler's handling of the texture load, dependent branches, and optimization-disabled path; done means both variants produce the expected green pixel.