[PATCH] PR63442 ICE with ubsan/overflow-int128.c test on AArch64

[Jiong Wang]

the cause should be one minor bug in prepare_cmp_insn.

the last mode parameter "pmode" of "prepare_cmp_insn" should match the
mode of the first parameter "x", while during the recursive call of 
"prepare_cmp_insn",
x is with mode of targetm.libgcc_cmp_return_mode () while pmode is assign to 
word_mode.

generally this is OK, because default libgcc_cmp_return_mode hook always return 
word_mode,
but AArch64 has a target private implementation which always return SImode, so 
there is a
mismatch which cause a ICE later.

this minor issue is hidding because nearly all other targets use default hook, 
and the
compare is rarely invoked.

Thanks

gcc/
  PR target/63442
  * optabs.c (prepare_cmp_insn): Use target hook "libgcc_cmp_return_mode" 
instead of word_mode.

diff --git a/gcc/optabs.c b/gcc/optabs.c
index d55a6bb..3073816 100644
--- a/gcc/optabs.c
+++ b/gcc/optabs.c
@@ -4264,7 +4264,7 @@ prepare_cmp_insn (rtx x, rtx y, enum rtx_code comparison, rtx size,
 	    y = const0_rtx;
 	}

-      *pmode = word_mode;
+      *pmode = targetm.libgcc_cmp_return_mode ();
       prepare_cmp_insn (x, y, comparison, NULL_RTX, unsignedp, methods,
 			ptest, pmode);
     }