[patch, tree-ssa] PR54295 Incorrect value extension in widening multiply-accumulate

[Richard Earnshaw <rearnsha at arm dot com>]

PR54295 shows that widening multiply-accumulate operations can end up
with incorrect code.  The path to the failure is as follows

1) Compiler detects a widening multiply operation and generates the
correct widening-multiply operation.  This involves stripping off the
widening cast to leave the underlying operands.

That is

	X = (c1) a * (c2) b
=>
	X = a w* b

the type of w is picked based on the types of a and b; if both are
signed a signed multiply-extend operation is used; if both are unsigned
an unsigned multiply-extend is used.  If they differ a signed/unsigned
multiply is used, if it exists.  If either a or b is a positive constant
it is coerced, if possible, into the type of the other operand.

2) The compiler then notices that the result, X is used in an addition
operation

	Y = X + d

As X is a widening multiply, the compiler then looks inside it to try
and generate a widening-multiply-accumulate operation.  However, it
passes the rewritten expression (X = a w* b) to is_widening_mult_p and
that routine does not correctly check what type of multiply is being
done: the code blindly tries to strip of an conversion operations.

The failure happens when a is also the result of a cast operation, for
example, being cast from an unsigned to an int.  The compiler then
re-formulates a signed multiply-extend into an unsigned multiply-extend.
 That is, if
	a = (int) e	// typeof(e) == unsigned

Then instead of
	Y = WIDEN_MULT_PLUS (a, b, d)
we end up with
	Y = WIDEN_MULT_PLUS (e, b, d)

The fix is to make is_widening_mult_p note that it has been called with
a WIDEN_MULT_EXPR and rather than decompose the operands again, to
simply extract the existing operands, which have already been formulated
correctly for a widening multiply operation.

	PR tree-ssa/54295
	* tree-ssa-math-opts.c (is_widening_mult_p): Short-circuit when
	the stmt is already a widening multiply.

Testsuite

	PR tree-ssa/54295
	* gcc.c-torture/execute/20120817-1.c: New test.

OK to commit once testing has completed?

Attachment: wmult.patch
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