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Fix __builtin_constant_p test in LTO mode

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LTO allows the compiler to prove variable values that non-LTO mode
cannot prove.  The workaround for the gcc-7 __builtin_constant_p
quirk incorrectly caused LTO on older gcc to report __builtin_constant_p
as unsupported.  Fix this by adding a stub asm() statement to hide the
value of the input parameter from the compiler.

Fixes: 1ed7cec714 ("Tighten __builtin_constant_p check to handle gcc-7")
This commit is contained in:
Kp 2017-08-02 02:49:12 +00:00
parent 9205296380
commit 7378179663
1 changed files with 25 additions and 1 deletions
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@ -1361,8 +1361,17 @@ help:assume compiler supports compile-time __builtin_constant_p
f = '''
/*
* Begin `timekeeping.i` from gcc bug #72785, comment #0. This block
* provokes gcc-7 into generating an impossible code path, which then
* provokes gcc-7 into generating a call to ____ilog2_NaN, which then
* fails to link when ____ilog2_NaN is intentionally undefined.
*
* Older versions of gcc declare the expression to be false in
* this case, and do not generate the call to ____ilog2_NaN. This looks
* fragile since b is provably non-zero (`b` = `a` if `a` is not zero,
* else = `c` = `1`, so `b` is non-zero regardless of the path used in
* the ternary operator), but the specific non-zero value is not
* provable. However, this matches the testcase posted in gcc's
* Bugzilla and produces the desired results on each of gcc-5 (enable),
* gcc-6 (enable), and gcc-7 (disable).
*/
int a, b;
int ____ilog2_NaN();
@ -1380,6 +1389,21 @@ static int x(int y){
}
'''
main = '''
/*
** Tell the compiler not to make any assumptions about the value of
** `a`. In LTO mode, the compiler would otherwise prove that it
** knows there are no writes to `a` and therefore prove that `a`
** must be zero. That proof then causes it to compile `by()` as:
**
** b = 0 ? 0 : 1;
** __builtin_constant_p(1) ? 1 ? ____ilog2_NaN() : 0 : 0;
**
** That then generates a reference to the intentionally undefined
** ____ilog2_NaN, causing the test to report failure. Prevent this
** by telling the compiler that it cannot prove the value of `a`,
** forcing it not to propagate a zero `a` into `by()`.
**/
asm("" : "=rm" (a));
by();
return x(1) + x(2);
'''