New IRgen of 'if' with constants is broken. #3432
Description
| Bugzilla Link | 3060 |
| Resolution | FIXED |
| Resolved on | Mar 06, 2010 13:59 |
| Version | unspecified |
| OS | All |
| Reporter | LLVM Bugzilla Contributor |
| CC | @lattner,@efriedma-quic,@nunoplopes,@regehr |
Extended Description
The new constant IRgen is too aggressive because it is possible to evaluate expressions with side effects to constants.
ddunbar@lordcrumb:CodeGen$ cat if.c
int g0(void);
int f0(void) {
if (g0() && 0)
return 0;
return 1;
}
ddunbar@lordcrumb:CodeGen$ clang -emit-llvm -o - -O3 if.c
; ModuleID = 'if.c'
target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128"
target triple = "i686-apple-darwin10.0.0d3"
define i32 @f0() nounwind readnone {
entry:
ret i32 1
}
The same issue occurs in other places, for example with the comma operator.
Chris and I discussed and came up with a plan to have tryEvaluate return more information about the folded constant. The idea would be to have the result be one of
(1) not evaluatable
(2) C99 ICE, which should never have side effects or labels
(3) constant value bla, with optional bits indicating if the expression has side effects and if the expression has labels (useful for consumers in different contexts). One can imagine other bits like imprecise-floating-point result being useful in time.
This bug is a placeholder for this work.