Handling of builtin_constant_p

Clang Frontend
1

Hi,

I'm trying to understand where __builtin_constant_p is folded in the
front-end. It appears to be handled differently when placed in an if
condition or in a conditional expression.

In the example below, I see that it is folded in
CodeGenFunction::EmitBuiltinExpr() in CGBuiltin.cpp
int x = __builtin_constant_p(100);

But for cases like the ones below, it appears to be folded much
earlier and it doesn't reach EmitBuiltinExpr().

int x = __builtin_constant_p(100)? 10: 20;

if (__builtin_constant_p(100)) { ... }

Any pointers on what functions to look up for the above case would be
very helpful.

Thanks,
Bharathi

2

grep -r "builtin_constant_p" under tool/clang shows some hints. For example, check VisitConditionalOperator if you want.

HTH,
chenwj

3

Hi,

I'm trying to understand where __builtin_constant_p is folded in the
front-end. It appears to be handled differently when placed in an if
condition or in a conditional expression.

In the example below, I see that it is folded in
CodeGenFunction::EmitBuiltinExpr() in CGBuiltin.cpp
int x = __builtin_constant_p(100);

But for cases like the ones below, it appears to be folded much
earlier and it doesn't reach EmitBuiltinExpr().

int x = __builtin_constant_p(100)? 10: 20;

if (__builtin_constant_p(100)) { ... }

Any pointers on what functions to look up for the above case would be
very helpful.

In some cases, Clang tries to constant-evaluate expressions rather than
emitting them directly. (C and C++ require this to be done in some cases
for variable initializers, particularly for globals, and Clang also tries
it when it emits some conditional branches, to avoid even emitting
trivially-unreachable code when the branch condition is constant). The
constant expression evaluation lives in lib/AST/ExprConstant.cpp, and knows
how to evaluate many of Clang's builtins (but not all of them).