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@@ -0,0 +1,76 @@
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+ Based on Richi's patch:
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+ <https://gcc.gnu.org/pipermail/gcc-patches/2024-April/648725.html>
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+ ~~
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+ The following avoids re-walking and re-combining the instructions
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+ between i2 and i3 when the pattern of i2 doesn't change.
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+
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+ Bootstrap and regtest running ontop of a reversal of
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+ r14-9692-g839bc42772ba7a.
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+
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+ It brings down memory use frmo 9GB to 400MB and compile-time from
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+ 80s to 3.5s. r14-9692-g839bc42772ba7a does better in both metrics
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+ but has shown code generation regressions across acrchitectures.
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+
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+ PR rtl-optimization/101523
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+ * combine.cc (try_combine): When the pattern of i2 doesn't
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+ change do not re-start combining at i2 or an earlier insn which
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+ had links or notes added.
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+ ~~
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+ But, since the patch affects code generation (for instance,
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+ libstdc++-v3/src/c++17/floating_from_chars.o), we limit the bailing out
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+ only when I2 hasn't been changed 1000x. I've measured how many times
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+ at most is I2 unchanged during a bootstrap + regtest.
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+ x86: 134
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+ aarch64: 736 (gimple-match-1.cc)
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+ s390x: 635 (gimple-match-*)
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+ ppc64le: 620 (gimple-match-*)
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+ while certain pathological testcases trigger it more than 10,000 times.
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+ With the limit in place this patch doesn't affect common code.
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+
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+ --- a/gcc/combine.cc
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+ +++ b/gcc/combine.cc
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+ @@ -92,6 +92,11 @@ along with GCC; see the file COPYING3. If not see
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+ #include "function-abi.h"
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+ #include "rtlanal.h"
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+
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+ +/* Number of times I2 didn't change in try_combine. Used to prevent a
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+ + combinatorial explosion. */
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+ +
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+ +static int combine_unchanged;
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+ +
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+ /* Number of attempts to combine instructions in this function. */
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+
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+ static int combine_attempts;
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+ @@ -1127,6 +1132,7 @@ combine_instructions (rtx_insn *f, unsigned int nregs)
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+ return false;
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+
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+ combine_attempts = 0;
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+ + combine_unchanged = 0;
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+ combine_merges = 0;
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+ combine_extras = 0;
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+ combine_successes = 0;
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+ @@ -4196,6 +4201,10 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0,
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+ adjust_for_new_dest (i3);
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+ }
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+
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+ + bool i2_unchanged = false;
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+ + if (rtx_equal_p (newi2pat, PATTERN (i2)))
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+ + i2_unchanged = true;
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+ +
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+ /* We now know that we can do this combination. Merge the insns and
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+ update the status of registers and LOG_LINKS. */
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+
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+ @@ -4762,6 +4771,13 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0,
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+ combine_successes++;
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+ undo_commit ();
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+
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+ + if (i2_unchanged)
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+ + {
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+ + if (combine_unchanged == 1000)
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+ + return i3;
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+ + ++combine_unchanged;
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+ + }
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+ +
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+ rtx_insn *ret = newi2pat ? i2 : i3;
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+ if (added_links_insn && DF_INSN_LUID (added_links_insn) < DF_INSN_LUID (ret))
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+ ret = added_links_insn;
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mpolacek
commented 16 days ago
Bootstrapped/regtested on x86_64-pc-linux-gnu.