[llvm-dev] [RFC] __builtin_constant_p() Improvements

[James Y Knight via llvm-dev]

I actually was working on an updated patch for the LLVM-side of this, also.
:) I was just working on some test cases; I'll post it soon. It's somewhat
different than yours.

I haven't touched the clang side yet, but I think it needs to be more
complex than what you have there. I think it actually needs to be able to
evaluate the intrinsic as a constant _false_ in the front-end in some
circumstances, rather than always emitting IR if it cannot tell the answer
is true.

On Thu, Apr 12, 2018 at 5:10 PM Bill Wendling via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello again!
>
> I took a stab at PR4898[1]. The attached patch improves Clang's
> __builtin_constant_p support so that the Linux kernel is happy. With this
> improvement, Clang can determine if __builtin_constant_p is true or false
> after inlining.
>
> As an example:
>
> static __attribute__((always_inline)) int foo(int x) {
> if (__builtin_constant_p(x))
> return 1;
> return 0;
> }
>
> static __attribute__((always_inline)) int mux() {
> if (__builtin_constant_p(37))
> return 927;
> return 0;
> }
>
> int bar(int a) {
> if (a)
> return foo(42);
> else
> return mux();
> }
>
> Now outputs this code at -O1:
>
> bar:
> .cfi_startproc
> # %bb.0:                                # %entry
> testl %edi, %edi
> movl $927, %ecx              # imm = 0x39F
> movl $1, %eax
> cmovel %ecx, %eax
> retq
>
> And this code at -O0:
>
> bar:                                    # @bar
> .cfi_startproc
> # %bb.0:                                # %entry
> pushq %rbp
> .cfi_def_cfa_offset 16
> .cfi_offset %rbp, -16
> movq %rsp, %rbp
> .cfi_def_cfa_register %rbp
> movl %edi, -16(%rbp)
> cmpl $0, -16(%rbp)
> je .LBB0_2
> # %bb.1:                                # %if.then
> movl $42, -8(%rbp)
> movl $0, -4(%rbp)
> movl -4(%rbp), %eax
> movl %eax, -12(%rbp)
> jmp .LBB0_3
> .LBB0_2:                                # %if.else
> movl $927, -12(%rbp)         # imm = 0x39F
> .LBB0_3:                                # %return
> movl -12(%rbp), %eax
> popq %rbp
> retq
>
> If the patch looks okay to people, I can shove it onto Phabricator for a
> review. (My phab-fu is bad.)
>
> Thoughts?
>
> -bw
>
> [1] https://bugs.llvm.org/show_bug.cgi?id=4898
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