Messages in this thread |  | | From | James Y Knight <> | Date | Thu, 05 Apr 2018 20:51:31 +0000 | Subject | Re: [GIT PULL] x86/build changes for v4.17 |
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On Thu, Apr 5, 2018 at 2:06 PM Linus Torvalds <torvalds@linux-foundation.org> wrote: > On Thu, Apr 5, 2018 at 10:46 AM, James Y Knight <jyknight@google.com> wrote: > > > > GCC, however, mixes up the concept of a C "constant expression" with the > > results of running optimization passes such as inlining for its > > definition/implementation of __builtin_constant_p. Clang does not, and quite > > likely will not ever, do that.
> Nothing has ever said that "__builtin_constant_p(x)" means "is x an > integer constant expression"
I had actually meant that the __builtin_constant_p **itself** had to be a constant expression, not that its *argument* must be an I-C-E for __builtin_constant_p to return true.
But after spending some time on further investigating in order to show an example of how this matters, I must take back my words. I was mistaken about GCC's semantics.
Take this example: === int function(void); void useval(int*);
int f() { int v = 1 + 2; int array[2][__builtin_constant_p(v) ? 1 : 100]; useval(array[0]); return sizeof(array[function()]) / sizeof(array[0]); } ===
Build with "gcc -O -std=c99": === f: subq $24, %rsp leaq 8(%rsp), %rdi call useval call function movl $4, %eax addq $24, %rsp ret ===
Note the fact that "function" is actually *called* indicates that 'array' is a VLA (...and that C99's sizeof(VLA) semantics are bonkers, but that's another story...).
Which means that __builtin_constant_p(v) was _not_ evaluated as an integer constant expression by GCC. Instead, it was left as an expression. And, the stack frame being only 24 bytes indicates that the __bcp eventually evaluated to true.
I actually think this actually _is_ something clang can implement. Thanks for making me try to prove myself. :)
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