[gcc r17-2413] PR middle-end/123236: Simplify (int)((long long)x >> 4)

https://gcc.gnu.org/g:e46d96d20bbfa786d30c9371631da00939845f05

commit r17-2413-ge46d96d20bbfa786d30c9371631da00939845f05
Author: Roger Sayle <[email protected]>
Date:   Wed Jul 15 12:26:16 2026 +0100

    PR middle-end/123236: Simplify (int)((long long)x >> 4)
    
    This patch addresses a code quality regression on x86_64 related to
    PR 123236.  That original PR (and the related PR 101266) concern tree
    level optimizations, where this problem should also be fixed, but it
    also reveals a regression in the RTL optimizers.
    
    A motivating test case (on x86_64) is:
    
    int bar(int a) {
      long long t = a;
      return t >> 4;
    }
    
    Currently -O2 generates a 64-bit shift:
    bar:    movslq  %edi, %rax
            sarq    $4, %rax
            ret
    
    with this patch we now generate a 32-bit shift:
    bar:    movl    %edi, %eax
            sarl    $4, %eax
            ret
    
    The underlying cause of the RTL-level regression is that some
    RTL expressions that were previously expressed as {ZERO,SIGN}_EXTEND
    are now sometimes represented as the equivalent {ZERO,SIGN}_EXTRACT,
    and that not all simplifications of TRUNCATE({ZERO,SIGN}_EXTEND) are
    implemented for TRUNCATE({ZERO,SIGN}_EXTRACT).
    
    Thanks to Segher, simplify_rtx does handle some truncations of extracts,
    see https://gcc.gnu.org/pipermail/gcc-patches/2016-November/463629.html
    but unfortunately this code (and subsequent tweaks) doesn't quite match
    the cases that appear here.
    
    2026-07-15  Roger Sayle  <[email protected]>
    
    gcc/ChangeLog
            PR rtl-optimization/123236
            * simplify-rtx.cc (simplify_context::simplify_truncation): Handle
            cases where a ZERO_EXTRACT or SIGN_EXTRACT has a different mode
            to (but at least as wide as) its first operand.
    
    gcc/testsuite/ChangeLog
            PR rtl-optimization/123236
            * gcc.target/i386/pr123236-1.c: New test case.

Diff:
---
 gcc/simplify-rtx.cc                        | 14 +++++++-------
 gcc/testsuite/gcc.target/i386/pr123236-1.c | 10 ++++++++++
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
index 92a2a6e954a8..882a11c5760d 100644
--- a/gcc/simplify-rtx.cc
+++ b/gcc/simplify-rtx.cc
@@ -727,12 +727,10 @@ simplify_context::simplify_truncation (machine_mode mode, rtx op,
 	}
     }
 
-  /* Turn (truncate:M1 (*_extract:M2 (reg:M2) (len) (pos))) into
-     (*_extract:M1 (truncate:M1 (reg:M2)) (len) (pos')) if possible without
-     changing len.  */
+  /* Turn (truncate:M1 (*_extract:M2 (reg:M3) (len) (pos))) into
+     (*_extract:M1 (truncate:M1 (reg:M3)) (len) (pos')) if possible.  */
   if ((GET_CODE (op) == ZERO_EXTRACT || GET_CODE (op) == SIGN_EXTRACT)
-      && REG_P (XEXP (op, 0))
-      && GET_MODE (XEXP (op, 0)) == GET_MODE (op)
+      && precision <= GET_MODE_UNIT_PRECISION (GET_MODE (XEXP (op, 0)))
       && CONST_INT_P (XEXP (op, 1))
       && CONST_INT_P (XEXP (op, 2)))
     {
@@ -741,7 +739,8 @@ simplify_context::simplify_truncation (machine_mode mode, rtx op,
       unsigned HOST_WIDE_INT pos = UINTVAL (XEXP (op, 2));
       if (BITS_BIG_ENDIAN && pos >= op_precision - precision)
 	{
-	  op0 = simplify_gen_unary (TRUNCATE, mode, op0, GET_MODE (op0));
+	  if (GET_MODE (op0) != mode)
+	    op0 = simplify_gen_unary (TRUNCATE, mode, op0, GET_MODE (op0));
 	  if (op0)
 	    {
 	      pos -= op_precision - precision;
@@ -751,7 +750,8 @@ simplify_context::simplify_truncation (machine_mode mode, rtx op,
 	}
       else if (!BITS_BIG_ENDIAN && precision >= len + pos)
 	{
-	  op0 = simplify_gen_unary (TRUNCATE, mode, op0, GET_MODE (op0));
+	  if (GET_MODE (op0) != mode)
+	    op0 = simplify_gen_unary (TRUNCATE, mode, op0, GET_MODE (op0));
 	  if (op0)
 	    return simplify_gen_ternary (GET_CODE (op), mode, mode, op0,
 					 XEXP (op, 1), XEXP (op, 2));
diff --git a/gcc/testsuite/gcc.target/i386/pr123236-1.c b/gcc/testsuite/gcc.target/i386/pr123236-1.c
new file mode 100644
index 000000000000..67e872f9b2d7
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr123236-1.c
@@ -0,0 +1,10 @@
+/* { dg-do compile { target { ! ia32 } } } */
+/* { dg-options "-O2" } */
+
+int foo(int a) {
+  long long t = a;
+  return t >> 4;
+}
+
+/* { dg-final { scan-assembler-not "movslq" } } */
+/* { dg-final { scan-assembler-not "sarq" } } */