I believe that a better (or supplementary) fix to PR target/107172 is to
avoid
producing incorrect (but valid) RTL in simplify_const_relational_operation
when
presented with questionable (obviously invalid) expressions, such as those
produced during combine. Just as with the "first do no harm" clause with
the
Hippocratic Oath, simplify-rtx (probably) shouldn't unintentionally
transform
invalid RTL expressions, into incorrect (non-equivalent) but valid RTL that
may be inappropriately recognized by recog.
In this specific case, many GCC backends represent their flags register via
MODE_CC, whose representation is intentionally "opaque" to the middle-end.
The only use of MODE_CC comprehensible to the middle-end's RTL optimizers
is relational comparisons between the result of a COMPARE rtx (op0) and zero
(op1). Any other uses of MODE_CC should be left alone, and some might argue
indicate representational issues in the backend.
In practice, CPUs occasionally have numerous instructions that affect the
flags register(s) other than comparisons [AVR's setc, powerpc's mtcrf,
x86's clc, stc and cmc and x86_64's ptest that sets C and Z flags in
non-obvious ways, c.f. PR target/109973]. Currently care has to be taken,
wrapping these in UNSPEC, to avoid combine inappropriately merging flags
setters with flags consumers (such as conditional jumps). It's safer to
teach simplify_const_relational_operation not to modify expressions that
it doesn't understand/recognize.
This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}
with no new failures. Ok for mainline?
2023-05-26 Roger Sayle <ro...@nextmovesoftware.com>
gcc/ChangeLog
* simplify-rtx.cc (simplify_const_relational_operation): Return
early
if we have a MODE_CC comparison that isn't a COMPARE against
const0_rtx.
Thanks in advance,
Roger
--
diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
index d4aeebc..d6444b4 100644
--- a/gcc/simplify-rtx.cc
+++ b/gcc/simplify-rtx.cc
@@ -6120,6 +6120,12 @@ simplify_const_relational_operation (enum rtx_code code,
|| (GET_MODE (op0) == VOIDmode
&& GET_MODE (op1) == VOIDmode));
+ /* We only handle MODE_CC comparisons that are COMPARE against zero. */
+ if (GET_MODE_CLASS (mode) == MODE_CC
+ && (op1 != const0_rtx
+ || GET_CODE (op0) != COMPARE))
+ return NULL_RTX;
+
/* If op0 is a compare, extract the comparison arguments from it. */
if (GET_CODE (op0) == COMPARE && op1 == const0_rtx)
{
