From: Jeff Law <jlaw@ventanamicro.com>
Date: Sat, 4 Oct 2025 14:33:19 +0000 (-0600)
Subject: [RISC-V][PR target/122147] Avoid creating (subreg (mem)) in RISC-V port
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=e037693f66823c168ed7d37ae70b3cd5aa757b1e;p=thirdparty%2Fgcc.git

[RISC-V][PR target/122147] Avoid creating (subreg (mem)) in RISC-V port

So another fun bug.  Utterly amazed we didn't trip over this in some form or
another until now.

We're generating a (subreg (mem)) expression during combine because
"move_operand" accepts it as a valid operand.  We've discouraged those kinds of
expressions for a long time, even though they're generally expected to act like
registers due to reloading.

In this case reloading just goes into an infinite loop 🙁   Rather than
try to fix this in LRA, let's just avoiding creating the problematical subreg
to begin with.  That's accomplished by being a bit more selective in what
move_operand allows.  I'm not particularly happy with what I saw in
move_operand, but I'm inclined to let it be right now.

Tested on rv32 and rv64.  Bootstraps on the Pioneer and BPI will run later
today.  I'll push once the pre-commit CI system has done its thing.

	PR target/122147
gcc/
	* config/riscv/predicates.md (move_operand): Only allow a REG as the
	operand of a SUBREG.

gcc/testsuite/

	* gcc.target/riscv/pr122147.c: New test.
---

diff --git a/gcc/config/riscv/predicates.md b/gcc/config/riscv/predicates.md
index 056f9e21909..f811a4e40ca 100644
--- a/gcc/config/riscv/predicates.md
+++ b/gcc/config/riscv/predicates.md
@@ -334,6 +334,13 @@
 	      && riscv_split_symbol_type (symbol_type)
 	      && symbol_type != SYMBOL_PCREL;
 
+    /* Be tight about the SUBREGs we accept.  In particular,
+       (subreg (mem)) has been discouraged for decades.  Just
+       allow (subreg (reg)) until such time as we see a strong
+       need to be more permissive.  */
+    case SUBREG:
+      return REG_P (SUBREG_REG (op));
+
     default:
       return true;
     }
diff --git a/gcc/testsuite/gcc.target/riscv/pr122147.c b/gcc/testsuite/gcc.target/riscv/pr122147.c
new file mode 100644
index 00000000000..14da321bc85
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/pr122147.c
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-additional-options "-w -march=rv64gcv -mabi=lp64d" { target rv64 } } */
+/* { dg-additional-options "-w -march=rv32gcv -mabi=ilp32" { target rv32 } } */
+
+typedef __attribute__((__vector_size__ (4))) _Float16 F;
+_Complex char cc;
+F f;
+
+void
+foo ()
+{
+  __builtin_memmove (&f, &cc, 2);
+  f *= f;
+}