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| author | Roger Sayle <roger@nextmovesoftware.com> | 2023-05-30 14:40:50 +0100 |
|---|---|---|
| committer | Roger Sayle <roger@nextmovesoftware.com> | 2023-05-30 14:43:27 +0100 |
| commit | 69185294f322dd53d4e1592115014c5488302e2e (patch) | |
| tree | d8c4110b42e7fb8d3b7715dedbee9107cde316cb | |
| parent | a899401404186843f38462c8fc9de733f19ce864 (diff) | |
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PR target/107172: Avoid "unusual" MODE_CC comparisons in simplify-rtx.cc
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.
2023-05-30 Roger Sayle <roger@nextmovesoftware.com>
gcc/ChangeLog
PR target/107172
* simplify-rtx.cc (simplify_const_relational_operation): Return
early if we have a MODE_CC comparison that isn't a COMPARE against
const0_rtx.
| -rw-r--r-- | gcc/simplify-rtx.cc | 6 |
1 files changed, 6 insertions, 0 deletions
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) { |