[WIP] Re-introduce 'TREE_USED' in tree streaming

[Thomas Schwinge <thomas@codesourcery.com>]

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Hi!

Now, that was another quirky debug session: in
'gcc/omp-low.cc:create_omp_child_function' we clearly do set
'TREE_USED (t) = 1;' for '.omp_data_i', which ends up as formal parameter
for outlined '[...]._omp_fn.[...]' functions, pointing to the "OMP blob".
Yet, in offloading compilation, I only ever got '!TREE_USED' for the
formal parameter '.omp_data_i'.  This greatly disturbs a nvptx back end
expand-time transformation that I have implemented, that's active
'if (!TREE_USED ([formal parameter]))'.

After checking along all the host-side OMP handling, eventually (in
hindsight: "obvious"...) I found that, "simply", we're not streaming
'TREE_USED'!  With that changed (see attached
"Re-introduce 'TREE_USED' in tree streaming"; no visible changes in
x86_64-pc-linux-gnu and powerpc64le-unknown-linux-gnu 'make check'), my
issue was quickly addressed -- if not for the question *why* 'TREE_USED'
isn't streamed (..., and apparently, that's a problem only for my
case..?), and then I found that it's *intentionally been removed*
in one-decade-old commit ee03e71d472a3f73cbc1a132a284309f36565972
(Subversion r200151) "Re-write LTO type merging again, do tree merging".

At this point, I need help: is this OK to re-introduce unconditionally,
or in some conditionalized form (but, "ugh..."), or be done differently
altogether in the nvptx back end (is 'TREE_USED' considered "stale" at
some point in the compilation pipeline?), or do we need some logic in
tree stream read-in (?) to achieve the same thing that removing
'TREE_USED' streaming apparently did achieve, or yet something else?
Indeed, from a quick look, most use of 'TREE_USED' seems to be "early",
but I saw no reason that it couldn't be used "late", either?

Original discussion "not streaming and comparing TREE_USED":
<https://inbox.sourceware.org/alpine.LNX.2.00.1306131614000.26078@zhemvz.fhfr.qr>
"[RFC] Re-write LTO type merging again, do tree merging", continued
<https://inbox.sourceware.org/alpine.LNX.2.00.1306141240340.6998@zhemvz.fhfr.qr>
"Re-write LTO type merging again, do tree merging".


In 2013, offloading compilation was just around the corner --
<https://inbox.sourceware.org/1375103926.7129.7694.camel@triegel.csb>
"Summary of the Accelerator BOF at Cauldron" -- and you easily could've
foreseen this issue, no?  ;-P


Grüße
 Thomas


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From cba6e4a8ec3b8718de7857b90d0137ae82f381fb Mon Sep 17 00:00:00 2001
From: Thomas Schwinge <thomas@codesourcery.com>
Date: Fri, 15 Sep 2023 00:14:13 +0200
Subject: [PATCH] [WIP] Re-introduce 'TREE_USED' in tree streaming

I have a nvptx back end expand-time transformation implemented, that's active
'if (!TREE_USED ([formal parameter]))'.  Now I found that per one-decade-old
commit ee03e71d472a3f73cbc1a132a284309f36565972 (Subversion r200151)
"Re-write LTO type merging again, do tree merging", 'TREE_USED' has
*intentionally been removed* from tree streaming.  That means, in nvptx
offloading compilation, every formal parameter (like for outlined
'[...]._omp_fn.[...]' functions the one that's pointing to the "OMP blob",
'.omp_data_i', for example) is considered unused, and thus mis-optimized.
---
 gcc/tree-streamer-in.cc  | 1 +
 gcc/tree-streamer-out.cc | 1 +
 2 files changed, 2 insertions(+)

diff --git a/gcc/tree-streamer-in.cc b/gcc/tree-streamer-in.cc
index 5bead0c3c6a..f82374e60a5 100644
--- a/gcc/tree-streamer-in.cc
+++ b/gcc/tree-streamer-in.cc
@@ -132,6 +132,7 @@ unpack_ts_base_value_fields (struct bitpack_d *bp, tree expr)
     TYPE_ARTIFICIAL (expr) = (unsigned) bp_unpack_value (bp, 1);
   else
     TREE_NO_WARNING (expr) = (unsigned) bp_unpack_value (bp, 1);
+  TREE_USED (expr) = (unsigned) bp_unpack_value (bp, 1);
   TREE_NOTHROW (expr) = (unsigned) bp_unpack_value (bp, 1);
   TREE_STATIC (expr) = (unsigned) bp_unpack_value (bp, 1);
   if (TREE_CODE (expr) != TREE_BINFO)
diff --git a/gcc/tree-streamer-out.cc b/gcc/tree-streamer-out.cc
index ff9694e17dd..74f969478cf 100644
--- a/gcc/tree-streamer-out.cc
+++ b/gcc/tree-streamer-out.cc
@@ -105,6 +105,7 @@ pack_ts_base_value_fields (struct bitpack_d *bp, tree expr)
     bp_pack_value (bp, TYPE_ARTIFICIAL (expr), 1);
   else
     bp_pack_value (bp, TREE_NO_WARNING (expr), 1);
+  bp_pack_value (bp, TREE_USED (expr), 1);
   bp_pack_value (bp, TREE_NOTHROW (expr), 1);
   bp_pack_value (bp, TREE_STATIC (expr), 1);
   if (TREE_CODE (expr) != TREE_BINFO)
-- 
2.34.1