HLSL compilation produces scalar-requiring cbuffer offsets by default

[baldurk]

Given this HLSL shader:

cbuffer consts
{
  float o[4];
  float p;
};

float4 main() : SV_Target0 { return p.xxxx; }

glslang used to emit a cbuffer with p at offset 64, which complies with the normal Vulkan packing rules. It looks like in #3575 this was changed to emit an offset of 52. Doing so requires scalar block layout on the vulkan side and to me is an unexpected behaviour change:

$ glslang -D -V -S frag -e main test.hlsl
$ spirv-val --target-env vulkan1.1spv1.4 frag.spv
error: line 27: Structure id 14 decorated as Block for variable in Uniform storage class must
follow relaxed uniform buffer layout rules: member1 at offset 52 overlaps previous member
ending at offset 63
  %consts = OpTypeStruct %_arr_float_uint_4 %float

$ spirv-val --target-env vulkan1.1spv1.4 --scalar-block-layout frag.spv

Compare to dxc which by default puts p at an offset of 64, but has options -fvk-use-dx-layout and -fvk-use-scalar-layout to emit D3D-packed layouts or scalar packed layouts respectively, though both require scalar block layout to remove this rule.

This effectively breaks backwards compatibility - was that deliberate? If so is there any way to at least opt-out to this behaviour if you don't want to have it opt-in?