doc: Document TARGET_TRULY_NOOP_TRUNCATION requires trunc?i?i2 patterns.

2020-07-16  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog:
	* target.def (TARGET_TRULY_NOOP_TRUNCATION): Clarify that
	targets that return false, indicating SUBREGs shouldn't be
	used, also need to provide a trunc?i?i2 optab that performs this
	truncation.
	* doc/tm.texi: Regenerate.

diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index 6e7d9dc54a9aecaea5a804cf1bca3eb74f27ea79..41b9e10c8561f408aacbc2c88447e226bd6b3a9e 100644 (file)
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -11122,7 +11122,9 @@ This hook returns true if it is safe to ``convert'' a value of
 @var{inprec} bits to one of @var{outprec} bits (where @var{outprec} is
 smaller than @var{inprec}) by merely operating on it as if it had only
 @var{outprec} bits.  The default returns true unconditionally, which
-is correct for most machines.
+is correct for most machines.  When @code{TARGET_TRULY_NOOP_TRUNCATION}
+returns false, the machine description should provide a @code{trunc}
+optab to specify the RTL that performs the required truncation.
 
 If @code{TARGET_MODES_TIEABLE_P} returns false for a pair of modes,
 suboptimal code can result if this hook returns true for the corresponding

diff --git a/gcc/target.def b/gcc/target.def
index 07059a87caf7cc0237d583124b476ee45ea41ed5..f2f314ed89c94aa527e89e33af8963ab1bf857ac 100644 (file)
--- a/gcc/target.def
+++ b/gcc/target.def
@@ -3148,7 +3148,9 @@ DEFHOOK
 @var{inprec} bits to one of @var{outprec} bits (where @var{outprec} is\n\
 smaller than @var{inprec}) by merely operating on it as if it had only\n\
 @var{outprec} bits.  The default returns true unconditionally, which\n\
-is correct for most machines.\n\
+is correct for most machines.  When @code{TARGET_TRULY_NOOP_TRUNCATION}\n\
+returns false, the machine description should provide a @code{trunc}\n\
+optab to specify the RTL that performs the required truncation.\n\
 \n\
 If @code{TARGET_MODES_TIEABLE_P} returns false for a pair of modes,\n\
 suboptimal code can result if this hook returns true for the corresponding\n\