[analyzer] assume bitwise arithmetic axioms

Patches the solver to assume that bitwise OR of an unsigned value with a
constant always produces a value larger-or-equal than the constant, and
bitwise AND with a constant always produces a value less-or-equal than
the constant.

This patch is especially useful in the context of using bitwise
arithmetic for error code encoding: the analyzer would be able to state
that the error code produced using a bitwise OR is non-zero.

Differential Revision: https://reviews.llvm.org/D39707

llvm-svn: 317820

1 files changed, 39 insertions, 0 deletions

diff --git a/clang/test/Analysis/constant-folding.c b/clang/test/Analysis/constant-folding.c
index a6d2b749427..8189868be2e 100644
--- a/clang/test/Analysis/constant-folding.c
+++ b/clang/test/Analysis/constant-folding.c
@@ -76,3 +76,42 @@ void testMixedTypeComparisons (char a, unsigned long b) {
   clang_analyzer_eval(b >= a); // expected-warning{{TRUE}}
   clang_analyzer_eval(a != b); // expected-warning{{TRUE}}
 }
+
+void testBitwiseRules(unsigned int a, int b) {
+  clang_analyzer_eval((a | 1) >= 1); // expected-warning{{TRUE}}
+  clang_analyzer_eval((a | -1) >= -1); // expected-warning{{TRUE}}
+  clang_analyzer_eval((a | 2) >= 2); // expected-warning{{TRUE}}
+  clang_analyzer_eval((a | 5) >= 5); // expected-warning{{TRUE}}
+  clang_analyzer_eval((a | 10) >= 10); // expected-warning{{TRUE}}
+
+  // Argument order should not influence this
+  clang_analyzer_eval((1 | a) >= 1); // expected-warning{{TRUE}}
+
+  clang_analyzer_eval((a & 1) <= 1); // expected-warning{{TRUE}}
+  clang_analyzer_eval((a & 2) <= 2); // expected-warning{{TRUE}}
+  clang_analyzer_eval((a & 5) <= 5); // expected-warning{{TRUE}}
+  clang_analyzer_eval((a & 10) <= 10); // expected-warning{{TRUE}}
+  clang_analyzer_eval((a & -10) <= 10); // expected-warning{{UNKNOWN}}
+
+  // Again, check for different argument order.
+  clang_analyzer_eval((1 & a) <= 1); // expected-warning{{TRUE}}
+
+  unsigned int c = a;
+  c |= 1;
+  clang_analyzer_eval((c | 0) == 0); // expected-warning{{FALSE}}
+
+  // Rules don't apply to signed typed, as the values might be negative.
+  clang_analyzer_eval((b | 1) > 0); // expected-warning{{UNKNOWN}}
+
+  // Even for signed values, bitwise OR with a non-zero is always non-zero.
+  clang_analyzer_eval((b | 1) == 0); // expected-warning{{FALSE}}
+  clang_analyzer_eval((b | -2) == 0); // expected-warning{{FALSE}}
+  clang_analyzer_eval((b | 10) == 0); // expected-warning{{FALSE}}
+  clang_analyzer_eval((b | 0) == 0); // expected-warning{{UNKNOWN}}
+  clang_analyzer_eval((b | -2) >= 0); // expected-warning{{UNKNOWN}}
+
+  // Check that dynamically computed constants also work.
+  int constant = 1 << 3;
+  unsigned int d = a | constant;
+  clang_analyzer_eval(constant > 0); // expected-warning{{TRUE}}
+}