| Message ID | 024501d9900a$682ef1c0$388cd540$@nextmovesoftware.com |
|---|---|
| State | New |
| Headers | show |
| Series | PR target/107172: Avoid "unusual" MODE_CC comparisons in simplify-rtx.cc | expand |
On 5/26/23 13:43, Roger Sayle wrote: > > 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 <roger@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. OK jeff
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) {
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 <roger@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 --