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tree-optimization/120032 - CLZ matching, fallback for missing range-info

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The following allows us to emit a conditional move when the value
of the table based CLZ/CLZ implementation at zero differs from what
the target implementation guarantees or we cannot easily fixup
otherwise.  In that case emit a val == 0 ? table-based-zero-result : ...

	PR tree-optimization/120032
	* tree-ssa-forwprop.cc (simplify_count_zeroes): When we cannot use
	the IFN to determine the result at zero use a conditional move
	to reproduce the correct result from the table-based
	algorithm.

	* gcc.target/i386/pr120032-3.c: New testcase.
This commit is contained in:
Richard Biener
2025-06-05 14:23:06 +02:00
parent c6eb92973e
commit 4a80eaefd9
2 changed files with 39 additions and 8 deletions
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20
gcc/testsuite/gcc.target/i386/pr120032-3.c Normal file
View File

@@ -0,0 +1,20 @@
/* { dg-do compile } */
/* { dg-options "-O2 -mlzcnt" } */
unsigned int
ZSTD_countLeadingZeros32_fallback(unsigned int val)
{
static const unsigned int DeBruijnClz[32]
= { 0, 9, 1, 10, 13, 21, 2, 29,
11, 14, 16, 18, 22, 25, 3, 30,
8, 12, 20, 28, 15, 17, 24, 7,
19, 27, 23, 6, 26, 5, 4, 31};
val |= val >> 1;
val |= val >> 2;
val |= val >> 4;
val |= val >> 8;
val |= val >> 16;
return 31 - DeBruijnClz[(val * 0x07C4ACDDU) >> 27];
}
/* { dg-final { scan-assembler "lzcnt" } } */

27
gcc/tree-ssa-forwprop.cc
View File

@@ -2728,13 +2728,6 @@ simplify_count_zeroes (gimple_stmt_iterator *gsi)
nargs = 1;
}
/* Skip if there is no value defined at zero, or if we can't easily
return the correct value for zero. */
if (!zero_ok)
return false;
if (zero_val != ctz_val && !(zero_val == 0 && ctz_val == input_bits))
return false;
gimple_seq seq = NULL;
gimple *g;
gcall *call = gimple_build_call_internal (fn, nargs, res_ops[0],
@@ -2758,8 +2751,10 @@ simplify_count_zeroes (gimple_stmt_iterator *gsi)
prev_lhs = gimple_assign_lhs (g);
}
if (zero_ok && zero_val == ctz_val)
;
/* Emit ctz (x) & 31 if ctz (0) is 32 but we need to return 0. */
if (zero_val == 0 && ctz_val == input_bits)
else if (zero_ok && zero_val == 0 && ctz_val == input_bits)
{
g = gimple_build_assign (make_ssa_name (integer_type_node),
BIT_AND_EXPR, prev_lhs,
@@ -2769,6 +2764,22 @@ simplify_count_zeroes (gimple_stmt_iterator *gsi)
gimple_seq_add_stmt (&seq, g);
prev_lhs = gimple_assign_lhs (g);
}
/* As fallback emit a conditional move. */
else
{
g = gimple_build_assign (make_ssa_name (boolean_type_node), EQ_EXPR,
res_ops[0], build_zero_cst (input_type));
gimple_set_location (g, gimple_location (stmt));
gimple_seq_add_stmt (&seq, g);
tree cond = gimple_assign_lhs (g);
g = gimple_build_assign (make_ssa_name (integer_type_node),
COND_EXPR, cond,
build_int_cst (integer_type_node, zero_val),
prev_lhs);
gimple_set_location (g, gimple_location (stmt));
gimple_seq_add_stmt (&seq, g);
prev_lhs = gimple_assign_lhs (g);
}
g = gimple_build_assign (gimple_assign_lhs (stmt), NOP_EXPR, prev_lhs);
gimple_seq_add_stmt (&seq, g);