i386: Improve code generation of smin(x,0) with -m32.

To make amends for the recent (temporary) testsuite failure
of my new gcc.target/i386/minmax-9.c when compiled with -m32,
this patch improves the -m32 code we generate for the examples
in that test case.

The trick is to expand smin(x,0) as "x < 0 ? x : 0" instead
of the current "x <= 0 ? x : 0", as the former can take
advantage of sign_bit_mask operations.

2020-08-10  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386-expand.c (ix86_expand_int_movcc): Expand
	signed MIN_EXPR against zero as "x < 0 ? x : 0" instead of
	"x <= 0 ? x : 0" to enable sign_bit_compare_p optimizations.

gcc/testsuite/ChangeLog
	* gcc.target/i386/minmax-12.c: New test.

diff --git a/gcc/config/i386/i386-expand.c b/gcc/config/i386/i386-expand.c
index 1bd0df4daf460ee5fa963a555b244c23363833b1..f441ba929bc178bc6f4534e915ca641f52f4ee49 100644 (file)
--- a/gcc/config/i386/i386-expand.c
+++ b/gcc/config/i386/i386-expand.c
@@ -3305,7 +3305,17 @@ ix86_expand_int_movcc (rtx operands[])
        {
          var = operands[2];
          if (INTVAL (operands[3]) == 0 && operands[2] != constm1_rtx)
-           operands[2] = constm1_rtx, op = and_optab;
+           {
+             /* For smin (x, 0), expand as "x < 0 ? x : 0" instead of
+                "x <= 0 ? x : 0" to enable sign_bit_compare_p.  */
+             if (code == LE && op1 == const0_rtx && rtx_equal_p (op0, var))
+               operands[1] = simplify_gen_relational (LT, VOIDmode,
+                                                      GET_MODE (op0),
+                                                      op0, const0_rtx);
+
+             operands[2] = constm1_rtx;
+             op = and_optab;
+           }
          else if (INTVAL (operands[3]) == -1 && operands[3] != const0_rtx)
            operands[2] = const0_rtx, op = ior_optab;
          else

diff --git a/gcc/testsuite/gcc.target/i386/minmax-12.c b/gcc/testsuite/gcc.target/i386/minmax-12.c
new file mode 100644 (file)
index 0000000..40efe54
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/minmax-12.c
@@ -0,0 +1,17 @@
+/* { dg-do compile { target ia32 } } */
+/* { dg-options "-O2 -march=i386 -mtune=generic" } */
+
+#define min(a,b) (((a) < (b))? (a) : (b))
+
+int foo(int x)
+{
+  return min(x,0);
+}
+
+signed char bar(signed char x)
+{
+  return min(x,0);
+}
+
+/* { dg-final { scan-assembler "cltd" } } */
+/* { dg-final { scan-assembler "sarb" } } */