diff --git a/gcc/ext-dce.cc b/gcc/ext-dce.cc
index 0ece37726c7..649d39fadf9 100644
--- a/gcc/ext-dce.cc
+++ b/gcc/ext-dce.cc
@@ -941,6 +941,38 @@ ext_dce_process_bb (basic_block bb)
     }
 }
 
+/* SUBREG_PROMOTED_VAR_P is set by the gimple->rtl optimizers and
+   is usually helpful.  However, in some cases setting the value when
+   it not strictly needed can cause this pass to miss optimizations.
+
+   Specifically consider (set (mem) (subreg (reg))).  If set in that
+   case it will cause more bit groups to be live for REG than would
+   be strictly necessary which in turn can inhibit extension removal.
+
+   So do a pass over the IL wiping the SUBREG_PROMOTED_VAR_P when it
+   is obviously not needed.  */
+
+static void
+maybe_clear_subreg_promoted_p (void)
+{
+  for (rtx_insn *insn = get_insns(); insn; insn = NEXT_INSN (insn))
+    {
+      if (!NONDEBUG_INSN_P (insn))
+	continue;
+
+      rtx set = single_set (insn);
+      if (!set)
+	continue;
+
+      /* There may be other cases where we should clear, but for
+	 now, this is the only known case where it causes problems.  */
+      if (MEM_P (SET_DEST (set)) && SUBREG_P (SET_SRC (set))
+        && GET_MODE (SET_DEST (set)) <= GET_MODE (SUBREG_REG (SET_SRC (set))))
+	SUBREG_PROMOTED_VAR_P (SET_SRC (set)) = 0;
+    }
+}
+
+
 /* We optimize away sign/zero extensions in this pass and replace
    them with SUBREGs indicating certain bits are don't cares.
 
@@ -1077,6 +1109,9 @@ static bool ext_dce_rd_confluence_n (edge) { return true; }
 void
 ext_dce_execute (void)
 {
+  /* Some settings of SUBREG_PROMOTED_VAR_P are actively harmful
+     to this pass.  Clear it for those cases.  */
+  maybe_clear_subreg_promoted_p ();
   df_analyze ();
   ext_dce_init ();
 
diff --git a/gcc/testsuite/gcc.target/riscv/ext-dce-1.c b/gcc/testsuite/gcc.target/riscv/ext-dce-1.c
new file mode 100644
index 00000000000..295d956ef55
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/ext-dce-1.c
@@ -0,0 +1,44 @@
+/* { dg-options "-O2 -fdump-rtl-ext_dce" } */
+typedef unsigned char __uint8_t;
+typedef unsigned int __uint32_t;
+typedef __uint8_t uint8_t;
+typedef __uint32_t uint32_t;
+static inline void
+unaligned_write32le(uint8_t *buf, uint32_t num)
+{
+ buf[0] = num;
+ buf[1] = num >> 8;
+ buf[2] = num >> 16;
+ buf[3] = num >> 24;
+ return;
+}
+typedef struct {
+ uint32_t dict_size;
+} lzma_options_lzma;
+typedef void lzma_coder;
+typedef struct lzma_next_coder_s lzma_next_coder;
+struct lzma_next_coder_s {
+ lzma_coder *coder;
+};
+struct lzma_coder_s {
+ uint8_t header[(1 + 4 + 8)];
+};
+
+void
+alone_encoder_init(lzma_next_coder *next, const lzma_options_lzma *options)
+{
+ uint32_t d = options->dict_size - 1;
+ d |= d >> 2;
+#if 0
+ d |= d >> 3;
+ d |= d >> 4;
+ d |= d >> 8;
+ d |= d >> 16;
+#endif
+ if (d != (4294967295U))
+  ++d;
+ unaligned_write32le(((struct lzma_coder_s*)next->coder)->header + 1, d);
+}
+
+/* { dg-final { scan-rtl-dump "Successfully transformed to:" "ext_dce" } } */
+