Optimization issue when specifying --target-env

[alexcher-im]

Hello, I noticed a performance degradations when calling some heavy non-side-effected GLSL function a few times, without using its result. Simplifying the code and checking that issue still exists, I ended up with this code:

#version 450
#extension GL_ARB_separate_shader_objects : enable
#extension GL_EXT_nonuniform_qualifier : enable

struct SampleStruct {
    int field;
};

layout (set = 0, binding = 1) uniform GlobalDSet0 {
    SampleStruct[16768] elems;
} buffers[];

SampleStruct read_buf() { return buffers[0].elems[0]; }

void main() {
    read_buf();
}

Which compiles (glslc shader.frag -O --target-env=vulkan1.2) into:

; SPIR-V
; Version: 1.5
; Generator: Google Shaderc over Glslang; 11
; Bound: 37
; Schema: 0
               OpCapability Shader
          %1 = OpExtInstImport "GLSL.std.450"
               OpMemoryModel Logical GLSL450
               OpEntryPoint Fragment %4 "main" %19
               OpExecutionMode %4 OriginUpperLeft
               OpMemberDecorate %_struct_11 0 Offset 0
               OpDecorate %_arr__struct_11_uint_16768 ArrayStride 16
               OpMemberDecorate %_struct_15 0 Offset 0
               OpDecorate %_struct_15 Block
               OpDecorate %19 DescriptorSet 0
               OpDecorate %19 Binding 1
       %void = OpTypeVoid
          %3 = OpTypeFunction %void
        %int = OpTypeInt 32 1
  %_struct_7 = OpTypeStruct %int
 %_struct_11 = OpTypeStruct %int
       %uint = OpTypeInt 32 0
 %uint_16768 = OpConstant %uint 16768
%_arr__struct_11_uint_16768 = OpTypeArray %_struct_11 %uint_16768
 %_struct_15 = OpTypeStruct %_arr__struct_11_uint_16768
     %uint_1 = OpConstant %uint 1
%_arr__struct_15_uint_1 = OpTypeArray %_struct_15 %uint_1
%_ptr_Uniform__arr__struct_15_uint_1 = OpTypePointer Uniform %_arr__struct_15_uint_1
         %19 = OpVariable %_ptr_Uniform__arr__struct_15_uint_1 Uniform
      %int_0 = OpConstant %int 0
%_ptr_Uniform__struct_11 = OpTypePointer Uniform %_struct_11
          %4 = OpFunction %void None %3
          %5 = OpLabel
         %34 = OpAccessChain %_ptr_Uniform__struct_11 %19 %int_0 %int_0 %int_0
         %35 = OpLoad %_struct_11 %34
         %36 = OpCopyLogical %_struct_7 %35
               OpReturn
               OpFunctionEnd

As I can see, read_buf() is a pure function, but calling it generates additional code in main().

This issue occures when I set --target-env to vulkan1.2 or vulkan1.3 , but clears out at vulkan1.0, vulkan1.1, or without specifying target env at all.

Version:
glslc from VulkanSDK 1.3.268.0

$ glslc --version
shaderc v2023.7 v2023.7
spirv-tools v2023.5 v2022.4-352-g360d469b
glslang 11.1.0-803-g48f9ed8b

Target: SPIR-V 1.0

Am I missing something about these Vulkan versions, or is this an optimizer bug?