clang: [[gnu::pure]] should mark ptr args as readonly

[qchateau]

For small enough return types (not sret), ptr arguments are readonly because the function is annotated memory(read):

struct NotSret {
    double x[2];
};

[[gnu::pure]] NotSret make_not_sret(void*);

; Function Attrs: mustprogress nofree nounwind willreturn memory(read)
declare dso_local { double, double } @_Z13make_not_sretPv(ptr noundef) local_unnamed_addr #39500 

For larger return types, if the function generates a sret argument, then the function is annotated memory(read, argmem: readwrite) and the optimizer cannot assume arguments other than the sret pointer cannot be written to. To leads to missed optimizations.

struct Sret {
    double x[3];
};

[[gnu::pure]] Sret make_sret(void*);

; Function Attrs: mustprogress nounwind willreturn memory(read, argmem: readwrite)
declare dso_local void @_Z9make_sretPv(ptr dead_on_unwind writable sret(%struct.Sret) align 8, ptr noundef) local_unnamed_addr #1
                                                                                                  ^ could add "readonly" here

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Quentin Chateau (qchateau)

For small enough return types (not sret), ptr arguments are readonly because the function is annotated memory(read):

struct NotSret {
    double x[2];
};

[[gnu::pure]] NotSret make_not_sret(void*);

; Function Attrs: mustprogress nofree nounwind willreturn memory(read)
declare dso_local { double, double } @<!-- -->_Z13make_not_sretPv(ptr noundef) local_unnamed_addr #<!-- -->39500 

For larger return types, if the function generates a sret argument, then the function is annotated memory(read, argmem: readwrite) and the optimizer cannot assume arguments other than the sret pointer cannot be written to. To leads to missed optimizations.

struct Sret {
    double x[3];
};

[[gnu::pure]] Sret make_sret(void*);

; Function Attrs: mustprogress nounwind willreturn memory(read, argmem: readwrite)
declare dso_local void @<!-- -->_Z9make_sretPv(ptr dead_on_unwind writable sret(%struct.Sret) align 8, ptr noundef) local_unnamed_addr #<!-- -->1
                                                                                                  ^ could add "readonly" here