From b9721bedf01ca1f536bbf13ba761333c6867bd29 Mon Sep 17 00:00:00 2001
From: whitequark <whitequark@whitequark.org>
Date: Mon, 21 Dec 2020 02:15:55 +0000
Subject: cxxrtl: speed up bit repeats (sign extends, etc).
On Minerva SoC SRAM, depending on the compiler, this change improves
overall time by 4-7%.
---
backends/cxxrtl/cxxrtl.h | 8 ++++++++
backends/cxxrtl/cxxrtl_backend.cc | 25 ++++++++++++++++++++-----
2 files changed, 28 insertions(+), 5 deletions(-)
diff --git a/backends/cxxrtl/cxxrtl.h b/backends/cxxrtl/cxxrtl.h
index 3c315c7df..0a6bcb849 100644
--- a/backends/cxxrtl/cxxrtl.h
+++ b/backends/cxxrtl/cxxrtl.h
@@ -317,6 +317,14 @@ struct value : public expr_base<value<Bits>> {
return sext_cast<NewBits>()(*this);
}
+ // Bit replication is far more efficient than the equivalent concatenation.
+ template<size_t Count>
+ CXXRTL_ALWAYS_INLINE
+ value<Bits * Count> repeat() const {
+ static_assert(Bits == 1, "repeat() is implemented only for 1-bit values");
+ return *this ? value<Bits * Count>().bit_not() : value<Bits * Count>();
+ }
+
// Operations with run-time parameters (offsets, amounts, etc).
//
// These operations are used for computations.
diff --git a/backends/cxxrtl/cxxrtl_backend.cc b/backends/cxxrtl/cxxrtl_backend.cc
index 3b2fb4985..916303bfe 100644
--- a/backends/cxxrtl/cxxrtl_backend.cc
+++ b/backends/cxxrtl/cxxrtl_backend.cc
@@ -832,11 +832,26 @@ struct CxxrtlWorker {
} else if (sig.is_chunk()) {
return dump_sigchunk(sig.as_chunk(), is_lhs, for_debug);
} else {
- dump_sigchunk(*sig.chunks().rbegin(), is_lhs, for_debug);
- for (auto it = sig.chunks().rbegin() + 1; it != sig.chunks().rend(); ++it) {
- f << ".concat(";
- dump_sigchunk(*it, is_lhs, for_debug);
- f << ")";
+ bool first = true;
+ auto chunks = sig.chunks();
+ for (auto it = chunks.rbegin(); it != chunks.rend(); it++) {
+ if (!first)
+ f << ".concat(";
+ bool is_complex = dump_sigchunk(*it, is_lhs, for_debug);
+ if (!is_lhs && it->width == 1) {
+ size_t repeat = 1;
+ while ((it + repeat) != chunks.rend() && *(it + repeat) == *it)
+ repeat++;
+ if (repeat > 1) {
+ if (is_complex)
+ f << ".val()";
+ f << ".repeat<" << repeat << ">()";
+ }
+ it += repeat - 1;
+ }
+ if (!first)
+ f << ")";
+ first = false;
}
return true;
}
--
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