glsl: Better IAdd Overflow CC fix

This ensures the original operand values are not overwritten when being used in the overflow detection.

src/shader_recompiler/backend/glsl/emit_glsl.cpp

@@ -227,7 +227,7 @@ std::string EmitGLSL(const Profile& profile, const RuntimeInfo& runtime_info, IR
     ctx.header += "void main(){\n";
     DefineVariables(ctx, ctx.header);
     if (ctx.uses_cc_carry) {
-        ctx.header += "uint carry;uint iadd_op_b;";
+        ctx.header += "uint carry;";
     }
     if (program.info.uses_subgroup_shuffles) {
         ctx.header += "bool shfl_in_bounds;";

src/shader_recompiler/backend/glsl/emit_glsl_integer.cpp

@@ -30,10 +30,21 @@ void SetSignFlag(EmitContext& ctx, IR::Inst& inst, std::string_view result) {
 } // Anonymous namespace
 
 void EmitIAdd32(EmitContext& ctx, IR::Inst& inst, std::string_view a, std::string_view b) {
+    // Compute the overflow CC first as it requires the original operand values,
+    // which may be overwritten by the result of the addition
+    if (IR::Inst * overflow{inst.GetAssociatedPseudoOperation(IR::Opcode::GetOverflowFromOp)}) {
+        // https://stackoverflow.com/questions/55468823/how-to-detect-integer-overflow-in-c
+        constexpr u32 s32_max{static_cast<u32>(std::numeric_limits<s32>::max())};
+        const auto sub_a{fmt::format("{}u-{}", s32_max, a)};
+        const auto positive_result{fmt::format("int({})>int({})", b, sub_a)};
+        const auto negative_result{fmt::format("int({})<int({})", b, sub_a)};
+        ctx.AddU1("{}=int({})>=0?{}:{};", *overflow, a, positive_result, negative_result);
+        overflow->Invalidate();
+    }
     const auto result{ctx.var_alloc.Define(inst, GlslVarType::U32)};
     if (IR::Inst* const carry{inst.GetAssociatedPseudoOperation(IR::Opcode::GetCarryFromOp)}) {
         ctx.uses_cc_carry = true;
-        ctx.Add("iadd_op_b={};{}=uaddCarry({},{},carry);", b, result, a, b);
+        ctx.Add("{}=uaddCarry({},{},carry);", result, a, b);
         ctx.AddU1("{}=carry!=0;", *carry);
         carry->Invalidate();
     } else {
@@ -41,15 +52,6 @@ void EmitIAdd32(EmitContext& ctx, IR::Inst& inst, std::string_view a, std::strin
     }
     SetZeroFlag(ctx, inst, result);
     SetSignFlag(ctx, inst, result);
-    if (IR::Inst * overflow{inst.GetAssociatedPseudoOperation(IR::Opcode::GetOverflowFromOp)}) {
-        // https://stackoverflow.com/questions/55468823/how-to-detect-integer-overflow-in-c
-        constexpr u32 s32_max{static_cast<u32>(std::numeric_limits<s32>::max())};
-        const auto sub_a{fmt::format("{}u-{}", s32_max, a)};
-        const auto op_b{ctx.uses_cc_carry ? "iadd_op_b" : b};
-        ctx.AddU1("{}=int({})>=0?int({})>int({}):int({})<int({});", *overflow, a, op_b, sub_a, op_b,
-                  sub_a);
-        overflow->Invalidate();
-    }
 }
 
 void EmitIAdd64(EmitContext& ctx, IR::Inst& inst, std::string_view a, std::string_view b) {