[PATCH v4] LOOP-UNROLL: Leverage HAS_SIGNED_ZERO for var expansion

[pan2.li@intel.com]

From: Pan Li <pan2.li@intel.com>

The insert_var_expansion_initialization depends on the
HONOR_SIGNED_ZEROS to initialize the unrolling variables
to +0.0f when -0.0f and no-signed-option.  Unfortunately,
we should always keep the -0.0f here because:

* The -0.0f is always the correct initial value.
* We need to support the target that always honor signed zero.

Thus, we need to leverage MODE_HAS_SIGNED_ZEROS when initialize
instead of HONOR_SIGNED_ZEROS.  Then the target/backend can
decide to honor the no-signed-zero or not.

The below tests are passed for this patch:

* The riscv regression tests.
* The aarch64 regression tests.
* The x86 bootstrap and regression tests.

gcc/ChangeLog:

	* loop-unroll.cc (insert_var_expansion_initialization): Leverage
	MODE_HAS_SIGNED_ZEROS for expansion variable initialization.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr30957-1.c: Adjust tests cases for different scenarios.

Signed-off-by: Pan Li <pan2.li@intel.com>
---
 gcc/loop-unroll.cc               |  4 +--
 gcc/testsuite/gcc.dg/pr30957-1.c | 48 ++++++++++++++++++++++++++++----
 2 files changed, 44 insertions(+), 8 deletions(-)

diff --git a/gcc/loop-unroll.cc b/gcc/loop-unroll.cc
index 4176a21e308..bfdfe6c2bb7 100644
--- a/gcc/loop-unroll.cc
+++ b/gcc/loop-unroll.cc
@@ -1855,7 +1855,7 @@ insert_var_expansion_initialization (struct var_to_expand *ve,
   rtx var, zero_init;
   unsigned i;
   machine_mode mode = GET_MODE (ve->reg);
-  bool honor_signed_zero_p = HONOR_SIGNED_ZEROS (mode);
+  bool has_signed_zero_p = MODE_HAS_SIGNED_ZEROS (mode);
 
   if (ve->var_expansions.length () == 0)
     return;
@@ -1869,7 +1869,7 @@ insert_var_expansion_initialization (struct var_to_expand *ve,
     case MINUS:
       FOR_EACH_VEC_ELT (ve->var_expansions, i, var)
         {
-	  if (honor_signed_zero_p)
+	  if (has_signed_zero_p)
 	    zero_init = simplify_gen_unary (NEG, mode, CONST0_RTX (mode), mode);
 	  else
 	    zero_init = CONST0_RTX (mode);
diff --git a/gcc/testsuite/gcc.dg/pr30957-1.c b/gcc/testsuite/gcc.dg/pr30957-1.c
index 564410913ab..6a9d3d87932 100644
--- a/gcc/testsuite/gcc.dg/pr30957-1.c
+++ b/gcc/testsuite/gcc.dg/pr30957-1.c
@@ -20,16 +20,52 @@ foo (float d, int n)
   return accum;
 }
 
+float __attribute__((noinline))
+get_minus_zero()
+{
+  return 0.0 / -5.0;
+}
+
 int
 main ()
 {
-  /* When compiling standard compliant we expect foo to return -0.0.  But the
-     variable expansion during unrolling optimization (for this testcase enabled
-     by non-compliant -fassociative-math) instantiates copy(s) of the
-     accumulator which it initializes with +0.0.  Hence we expect that foo
-     returns +0.0.  */
-  if (__builtin_copysignf (1.0, foo (0.0 / -5.0, 10)) != 1.0)
+  /* The variable expansion in unroll requires option unsafe-math-optimizations
+     (aka -fno-signed-zeros, -fno-trapping-math, -fassociative-math
+     and -freciprocal-math).
+
+     When loop like above will have expansion after unrolling as below:
+
+     accum_1 += d_1;
+     accum_2 += d_2;
+     accum_3 += d_3;
+     ...
+
+     The accum_1, accum_2 and accum_3 need to be initialized. Given the
+     floating-point we have
+     +0.0f + -0.0f = +0.0f.
+
+     Thus, we should initialize the accum_* to -0.0 for correctness.  But
+     the things become more complicated when no-signed-zeros, as well as VLA
+     vectorizer mode which doesn't trigger variable expansion. Then we have:
+
+     Case 1: Trigger variable expansion but target doesn't honor no-signed-zero.
+       minus_zero will be -0.0f and foo (minus_zero, 10) will be -0.0f.
+     Case 2: Trigger variable expansion but target does honor no-signed-zero.
+       minus_zero will be +0.0f and foo (minus_zero, 10) will be +0.0f.
+     Case 3: No variable expansion but target doesn't honor no-signed-zero.
+       minus_zero will be -0.0f and foo (minus_zero, 10) will be -0.0f.
+     Case 4: No variable expansion but target does honor no-signed-zero.
+       minus_zero will be +0.0f and foo (minus_zero, 10) will be +0.0f.
+
+     The test case covers above 4 cases for running.
+     */
+  float minus_zero = get_minus_zero ();
+  float a = __builtin_copysignf (1.0, minus_zero);
+  float b = __builtin_copysignf (1.0, foo (minus_zero, 10));
+
+  if (a != b)
     abort ();
+
   exit (0);
 }
 
-- 
2.34.1