[PATCH] libcpp: Fix unsigned promotion for unevaluated divide by zero [PR112701]

[PATCH] libcpp: Fix unsigned promotion for unevaluated divide by zero [PR112701]

[Lewis Hyatt]

Hello-

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112701

Here is a one-line fix to an edge case in libcpp's expression evaluator
noted in the PR. Bootstrap + regtest all languages on x86-64 Linux. Is it OK
please? Thanks!

-Lewis

-- >8 --

When libcpp encounters a divide by zero while processing a constant
expression "x/y", it returns "x" as a fallback. The value of the fallback is
not normally important, since an error will be generated anyway, but if the
expression appears in an unevaluated context, such as "0 ? 0/0u : -1", then
there will be no error, and the fallback value will be meaningful to the
extent that it may cause promotion from signed to unsigned of an operand
encountered later. As the PR notes, libcpp does not do the unsigned
promotion correctly in this case; fix it by making the fallback return value
unsigned as necessary.

libcpp/ChangeLog:

	PR preprocessor/112701
	* expr.cc (num_div_op): Set unsignedp appropriately when returning a
	stub value for divide by 0.

gcc/testsuite/ChangeLog:

	PR preprocessor/112701
	* gcc.dg/cpp/expr.c: Add additional tests to cover divide by 0 in an
	unevaluated context, where the unsignedness still matters.
---
 libcpp/expr.cc                  |  1 +
 gcc/testsuite/gcc.dg/cpp/expr.c | 22 ++++++++++++++++++++--
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/libcpp/expr.cc b/libcpp/expr.cc
index 825d2c2369d..4f4a9722ac7 100644
--- a/libcpp/expr.cc
+++ b/libcpp/expr.cc
@@ -2216,6 +2216,7 @@ num_div_op (cpp_reader *pfile, cpp_num lhs, cpp_num rhs, enum cpp_ttype op,
       if (!pfile->state.skip_eval)
 	cpp_error_with_line (pfile, CPP_DL_ERROR, location, 0,
 			     "division by zero in #if");
+      lhs.unsignedp = unsignedp;
       return lhs;
     }
 
diff --git a/gcc/testsuite/gcc.dg/cpp/expr.c b/gcc/testsuite/gcc.dg/cpp/expr.c
index 532bd681237..055e17ae753 100644
--- a/gcc/testsuite/gcc.dg/cpp/expr.c
+++ b/gcc/testsuite/gcc.dg/cpp/expr.c
@@ -1,6 +1,7 @@
 /* Copyright (C) 2000, 2001 Free Software Foundation, Inc.  */
 
 /* { dg-do preprocess } */
+/* { dg-additional-options "-Wall" } */
 
 /* Test we get signedness of ?: operator correct.  We would skip
    evaluation of one argument, and might therefore not transfer its
@@ -8,10 +9,27 @@
 
 /* Neil Booth, 19 Jul 2002.  */
 
-#if (1 ? -2: 0 + 1U) < 0
+#if (1 ? -2: 0 + 1U) < 0 /* { dg-warning {the left operand of ":" changes sign} } */
 #error				/* { dg-bogus "error" } */
 #endif
 
-#if (0 ? 0 + 1U: -2) < 0
+#if (0 ? 0 + 1U: -2) < 0 /* { dg-warning {the right operand of ":" changes sign} } */
 #error				/* { dg-bogus "error" } */
 #endif
+
+/* PR preprocessor/112701 */
+#if (0 ? 0/0u : -1) < 0 /* { dg-warning {the right operand of ":" changes sign} } */
+#error /* { dg-bogus "error" } */
+#endif
+
+#if (0 ? 0u/0 : -1) < 0 /* { dg-warning {the right operand of ":" changes sign} } */
+#error /* { dg-bogus "error" } */
+#endif
+
+#if (1 ? -1 : 0/0u) < 0 /* { dg-warning {the left operand of ":" changes sign} } */
+#error /* { dg-bogus "error" } */
+#endif
+
+#if (1 ? -1 : 0u/0) < 0 /* { dg-warning {the left operand of ":" changes sign} } */
+#error /* { dg-bogus "error" } */
+#endif

Re: [PATCH] libcpp: Fix unsigned promotion for unevaluated divide by zero [PR112701]

[Joseph Myers]

On Mon, 27 Nov 2023, Lewis Hyatt wrote:

> Hello-
> 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112701
> 
> Here is a one-line fix to an edge case in libcpp's expression evaluator
> noted in the PR. Bootstrap + regtest all languages on x86-64 Linux. Is it OK
> please? Thanks!

OK.

-- 
Joseph S. Myers
joseph@codesourcery.com