[PATCH] PR 91865: Avoid ZERO_EXTEND of ZERO_EXTEND in make_compound_operation.

[Roger Sayle]

This patch is my proposed solution to PR rtl-optimization/91865.
Normally RTX simplification canonicalizes a ZERO_EXTEND of a ZERO_EXTEND
to a single ZERO_EXTEND, but as shown in this PR it is possible for
combine's make_compound_operation to unintentionally generate a
non-canonical ZERO_EXTEND of a ZERO_EXTEND, which is unlikely to be
matched by the backend.

For the new test case:

const int table[2] = {1, 2};
int foo (char i) { return table[i]; }

compiling with -O2 -mlarge on msp430 we currently see:

Trying 2 -> 7:
    2: r25:HI=zero_extend(R12:QI)
      REG_DEAD R12:QI
    7: r28:PSI=sign_extend(r25:HI)#0
      REG_DEAD r25:HI
Failed to match this instruction:
(set (reg:PSI 28 [ iD.1772 ])
    (zero_extend:PSI (zero_extend:HI (reg:QI 12 R12 [ iD.1772 ]))))

which results in the following code:

foo:    AND     #0xff, R12
        RLAM.A #4, R12 { RRAM.A #4, R12
        RLAM.A  #1, R12
        MOVX.W  table(R12), R12
        RETA

With this patch, we now see:

Trying 2 -> 7:
    2: r25:HI=zero_extend(R12:QI)
      REG_DEAD R12:QI
    7: r28:PSI=sign_extend(r25:HI)#0
      REG_DEAD r25:HI
Successfully matched this instruction:
(set (reg:PSI 28 [ iD.1772 ])
    (zero_extend:PSI (reg:QI 12 R12 [ iD.1772 ])))
allowing combination of insns 2 and 7
original costs 4 + 8 = 12
replacement cost 8

foo:    MOV.B   R12, R12
        RLAM.A  #1, R12
        MOVX.W  table(R12), R12
        RETA


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?

2023-10-14  Roger Sayle  <ro...@nextmovesoftware.com>

gcc/ChangeLog
        PR rtl-optimization/91865
        * combine.cc (make_compound_operation): Avoid creating a
        ZERO_EXTEND of a ZERO_EXTEND.

gcc/testsuite/ChangeLog
        PR rtl-optimization/91865
        * gcc.target/msp430/pr91865.c: New test case.


Thanks in advance,
Roger
--


diff --git a/gcc/combine.cc b/gcc/combine.cc
index 360aa2f25e6..f47ff596782 100644
--- a/gcc/combine.cc
+++ b/gcc/combine.cc
@@ -8453,6 +8453,9 @@ make_compound_operation (rtx x, enum rtx_code in_code)
                                            new_rtx, GET_MODE (XEXP (x, 0)));
       if (tem)
        return tem;
+      /* Avoid creating a ZERO_EXTEND of a ZERO_EXTEND.  */
+      if (GET_CODE (new_rtx) == ZERO_EXTEND)
+       new_rtx = XEXP (new_rtx, 0);
       SUBST (XEXP (x, 0), new_rtx);
       return x;
     }
diff --git a/gcc/testsuite/gcc.target/msp430/pr91865.c 
b/gcc/testsuite/gcc.target/msp430/pr91865.c
new file mode 100644
index 00000000000..8cc21c8b9e8
--- /dev/null
+++ b/gcc/testsuite/gcc.target/msp430/pr91865.c
@@ -0,0 +1,8 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -mlarge" } */
+
+const int table[2] = {1, 2};
+int foo (char i) { return table[i]; }
+
+/* { dg-final { scan-assembler-not "AND" } } */
+/* { dg-final { scan-assembler-not "RRAM" } } */