[gcc/devel/gccgo] middle-end: Improve RTL expansion in expand_mul_overflow,

[Ian Lance Taylor]

https://gcc.gnu.org/g:2b4bbc17077ac12fc9742ade4919a6de0277ac8c

commit 2b4bbc17077ac12fc9742ade4919a6de0277ac8c
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Sat Jul 11 20:03:39 2020 +0100

    middle-end: Improve RTL expansion in expand_mul_overflow,
    
    This patch improves the RTL that the middle-end generates for testing
    signed overflow following a widening multiplication.  During this
    expansion the middle-end generates a truncation which can get used
    multiple times.  Placing this intermediate value in a pseudo register
    reduces the amount of code generated on platforms where this truncation
    requires an explicit instruction.
    
    2020-07-11  Roger Sayle  <roger@nextmovesoftware.com>
    
    gcc/ChangeLog:
            * internal-fn.c (expand_mul_overflow): When checking for signed
            overflow from a widening multiplication, we access the truncated
            lowpart RES twice, so keep this value in a pseudo register.

Diff:
---
 gcc/internal-fn.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/gcc/internal-fn.c b/gcc/internal-fn.c
index a15031f794d..8efc77d986b 100644
--- a/gcc/internal-fn.c
+++ b/gcc/internal-fn.c
@@ -1629,6 +1629,9 @@ expand_mul_overflow (location_t loc, tree lhs, tree arg0, tree arg1,
 				     profile_probability::very_likely ());
 	  else
 	    {
+	      /* RES is used more than once, place it in a pseudo.  */
+	      res = force_reg (mode, res);
+
 	      rtx signbit = expand_shift (RSHIFT_EXPR, mode, res, prec - 1,
 					  NULL_RTX, 0);
 	      /* RES is low half of the double width result, HIPART