[gcc r12-3251] Only simplify TRUNCATE to SUBREG on TRULY_NOOP_TRUNCATION targets.

[gcc r12-3251] Only simplify TRUNCATE to SUBREG on TRULY_NOOP_TRUNCATION targets.

[Roger Sayle]

https://gcc.gnu.org/g:0960d937d9bee3c831d0b64a9c828c263a58ff89

commit r12-3251-g0960d937d9bee3c831d0b64a9c828c263a58ff89
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Tue Aug 31 11:41:57 2021 +0100

    Only simplify TRUNCATE to SUBREG on TRULY_NOOP_TRUNCATION targets.
    
    As recently remarked by Jeff Law, SUBREGs are the "forever chemicals"
    of GCC's RTL; once created they persist in the environment.  The problem,
    according to the comment on lines 5428-5438 of combine.c is that
    non-tieable SUBREGs interfere with reload/register allocation, so
    combine often doesn't touch/clean-up instructions containing a SUBREG.
    
    This is the first and simplest of two patches to tackle that problem,
    by teaching combine to avoid converting explicit TRUNCATEs into
    SUBREGs that it can't handle.
    
    Consider the following (hypothetical) sequence of instructions on
    a STORE_FLAG_VALUE=1 target, which stores a zero or one in an SI
    register, then uselessly truncates to QImode, then extends it again.
    
    (set (reg:SI 27) (ne:SI (reg:BI 28) (const_int 0)))
    (set (reg:QI 26) (truncate:QI (reg:SI 27)))
    (set (reg:SI 0) (zero_extend:SI (reg:QI 26)))
    
    which ideally (i.e. with this patch) combine would simplify to:
    (set (reg:SI 0) (ne:SI (reg:BI 28) (const_int 0)))
    
    Alas currently, during combine the middle TRUNCATE is converted into
    a lowpart SUBREG, which subst then turns into (clobber (const_int 0)),
    abandoning the attempted combination, that then never reaches recog.
    
    2021-08-31  Roger Sayle  <roger@nextmovesoftware.com>
    
    gcc/ChangeLog
            * combine.c (combine_simplify_rtx): Avoid converting an explicit
            TRUNCATE into a lowpart SUBREG on !TRULY_NOOP_TRUNCATION targets.
            * simplify-rtx.c (simplify_unary_operation_1): Likewise.

Diff:
---
 gcc/combine.c      | 3 ++-
 gcc/simplify-rtx.c | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/gcc/combine.c b/gcc/combine.c
index cb5fa401fcb..290a3667c65 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -5903,7 +5903,8 @@ combine_simplify_rtx (rtx x, machine_mode op0_mode, int in_dest,
       if (HWI_COMPUTABLE_MODE_P (mode)
 	  && (STORE_FLAG_VALUE & ~GET_MODE_MASK (mode)) == 0
 	  && (temp = get_last_value (XEXP (x, 0)))
-	  && COMPARISON_P (temp))
+	  && COMPARISON_P (temp)
+	  && TRULY_NOOP_TRUNCATION_MODES_P (mode, GET_MODE (XEXP (x, 0))))
 	return gen_lowpart (mode, XEXP (x, 0));
       break;
 
diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index c81e27eb008..e431e0c19d7 100644
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -1249,7 +1249,8 @@ simplify_context::simplify_unary_operation_1 (rtx_code code, machine_mode mode,
          than HOST_BITS_PER_WIDE_INT.  */
       if (HWI_COMPUTABLE_MODE_P (mode)
 	  && COMPARISON_P (op)
-	  && (STORE_FLAG_VALUE & ~GET_MODE_MASK (mode)) == 0)
+	  && (STORE_FLAG_VALUE & ~GET_MODE_MASK (mode)) == 0
+	  && TRULY_NOOP_TRUNCATION_MODES_P (mode, GET_MODE (op)))
 	{
 	  temp = rtl_hooks.gen_lowpart_no_emit (mode, op);
 	  if (temp)