[PATCH] arm: Fix ICE due to infinite splitting [PR109800]

[Alex Coplan via Gcc-patches]

Hi,

In r11-966-g9a182ef9ee011935d827ab5c6c9a7cd8e22257d8 we introduce a
simplification to emit_move_insn that attempts to simplify moves of the form:

(set (subreg:M1 (reg:M2 ...)) (constant C))

where M1 and M2 are of equal mode size. That is problematic for the splitter
vfp.md:no_literal_pool_df_immediate in the arm backend, which tries to pun an
lvalue DFmode pseudo into DImode and assign a constant to it with
emit_move_insn, as the new transformation simply undoes this, and we end up
splitting indefinitely.

This patch changes things around in the arm backend so that we use a
DImode temporary (instead of DFmode) and first load the DImode constant
into the pseudo, and then pun the pseudo into DFmode as an rvalue in a
reg -> reg move. I believe this should be semantically equivalent but
avoids the pathalogical behaviour seen in the PR.

Bootstrapped/regtested on arm-linux-gnueabihf, regtested on
arm-none-eabi and armeb-none-eabi.

OK for trunk and backports?

Thanks,
Alex

gcc/ChangeLog:

        PR target/109800
        * config/arm/arm.md (movdf): Generate temporary pseudo in DImode
        instead of DFmode.
        * config/arm/vfp.md (no_literal_pool_df_immediate): Rather than punning 
an
        lvalue DFmode pseudo into DImode, use a DImode pseudo and pun it into
        DFmode as an rvalue.

gcc/testsuite/ChangeLog:

        PR target/109800
        * gcc.target/arm/pr109800.c: New test.

diff --git a/gcc/config/arm/arm.md b/gcc/config/arm/arm.md
index cbfc4543531..40c4d848238 100644
--- a/gcc/config/arm/arm.md
+++ b/gcc/config/arm/arm.md
@@ -7555,7 +7555,7 @@ (define_expand "movdf"
       && !arm_const_double_rtx (operands[1])
       && !(TARGET_VFP_DOUBLE && vfp3_const_double_rtx (operands[1])))
     {
-      rtx clobreg = gen_reg_rtx (DFmode);
+      rtx clobreg = gen_reg_rtx (DImode);
       emit_insn (gen_no_literal_pool_df_immediate (operands[0], operands[1],
                                                   clobreg));
       DONE;
diff --git a/gcc/config/arm/vfp.md b/gcc/config/arm/vfp.md
index 60e7ba35d8b..03514acc94f 100644
--- a/gcc/config/arm/vfp.md
+++ b/gcc/config/arm/vfp.md
@@ -2138,7 +2138,7 @@ (define_insn "get_fpscr"
 (define_insn_and_split "no_literal_pool_df_immediate"
   [(set (match_operand:DF 0 "s_register_operand" "=w")
        (match_operand:DF 1 "const_double_operand" "F"))
-   (clobber (match_operand:DF 2 "s_register_operand" "=r"))]
+   (clobber (match_operand:DI 2 "s_register_operand" "=r"))]
   "arm_disable_literal_pool
    && TARGET_VFP_BASE
    && !arm_const_double_rtx (operands[1])
@@ -2153,8 +2153,9 @@ (define_insn_and_split "no_literal_pool_df_immediate"
   unsigned HOST_WIDE_INT ival = zext_hwi (buf[order], 32);
   ival |= (zext_hwi (buf[1 - order], 32) << 32);
   rtx cst = gen_int_mode (ival, DImode);
-  emit_move_insn (simplify_gen_subreg (DImode, operands[2], DFmode, 0), cst);
-  emit_move_insn (operands[0], operands[2]);
+  emit_move_insn (operands[2], cst);
+  emit_move_insn (operands[0],
+                 simplify_gen_subreg (DFmode, operands[2], DImode, 0));
   DONE;
 }
 )
diff --git a/gcc/testsuite/gcc.target/arm/pr109800.c 
b/gcc/testsuite/gcc.target/arm/pr109800.c
new file mode 100644
index 00000000000..71d1ede13dd
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/pr109800.c
@@ -0,0 +1,3 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -march=armv7-m -mfloat-abi=hard -mfpu=fpv4-sp-d16 
-mbig-endian -mpure-code" } */
+double f() { return 5.0; }