https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87817
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jakub at gcc dot gnu.org
--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The important question is if ZERO_EXTRACT is well defined with zero SIZE and
what it means. And another question is if SIGN_EXTRACT is well defined with
zero SIZE and if yes, how would we define it.
If it is defined, we could do something like:
--- gcc/combine.c.jj 2018-11-12 16:18:28.656552899 +0100
+++ gcc/combine.c 2018-11-13 13:08:38.776716046 +0100
@@ -7271,6 +7271,15 @@ expand_compound_operation (rtx x)
if (len + pos > GET_MODE_PRECISION (inner_mode))
return x;
+ if (len == 0)
+ {
+ /* Extracting no bits for ZERO_EXTRACT always yields 0. */
+ if (unsignedp)
+ return CONST0_RTX (GET_MODE (x));
+ /* For SIGN_EXTRACT it is undefined. */
+ return x;
+ }
+
if (BITS_BIG_ENDIAN)
pos = GET_MODE_PRECISION (inner_mode) - len - pos;
which fixes this PR, but we have other spots that would need tweaking, e.g.
simplify-rtx.c, another spot in the combiner etc.
Another possibility is to say that both are UB with zero SIZE and define the
pattern differently.
