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Richard.
2015-09-14 Richard Biener <rguent...@suse.de>
* doc/match-and-simplify.texi: Update for changed syntax
of inner ifs and the new switch expression.
Index: gcc/doc/match-and-simplify.texi
===================================================================
--- gcc/doc/match-and-simplify.texi (revision 227739)
+++ gcc/doc/match-and-simplify.texi (working copy)
@@ -118,8 +118,8 @@ be a valid GIMPLE operand (so you cannot
@smallexample
(simplify
(trunc_mod integer_zerop@@0 @@1)
- (if (!integer_zerop (@@1)))
- @@0)
+ (if (!integer_zerop (@@1))
+ @@0))
@end smallexample
Here @code{@@0} captures the first operand of the trunc_mod expression
@@ -130,9 +130,11 @@ can be unconstrained or capture expresio
This example introduces an optional operand of simplify,
the if-expression. This condition is evaluated after the
expression matched in the IL and is required to evaluate to true
-to enable the replacement expression. The expression operand
-of the @code{if} is a standard C expression which may contain references
-to captures.
+to enable the replacement expression in the second operand
+position. The expression operand of the @code{if} is a standard C
+expression which may contain references to captures. The @code{if}
+has an optional third operand which may contain the replacement
+expression that is enabled when the condition evaluates to false.
A @code{if} expression can be used to specify a common condition
for multiple simplify patterns, avoiding the need
@@ -149,8 +151,48 @@ to repeat that multiple times:
(negate @@1)))
@end smallexample
+Note that @code{if}s in outer position do not have the optional
+else clause but instead have multiple then clauses.
+
Ifs can be nested.
+There exists a @code{switch} expression which can be used to
+chain conditions avoiding nesting @code{if}s too much:
+
+@smallexample
+(simplify
+ (simple_comparison @@0 REAL_CST@@1)
+ (switch
+ /* a CMP (-0) -> a CMP 0 */
+ (if (REAL_VALUE_MINUS_ZERO (TREE_REAL_CST (@@1)))
+ (cmp @@0 @{ build_real (TREE_TYPE (@@1), dconst0); @}))
+ /* x != NaN is always true, other ops are always false. */
+ (if (REAL_VALUE_ISNAN (TREE_REAL_CST (@@1))
+ && ! HONOR_SNANS (@@1))
+ @{ constant_boolean_node (cmp == NE_EXPR, type); @})))
+@end smallexample
+
+Is equal to
+
+@smallexample
+(simplify
+ (simple_comparison @@0 REAL_CST@@1)
+ (switch
+ /* a CMP (-0) -> a CMP 0 */
+ (if (REAL_VALUE_MINUS_ZERO (TREE_REAL_CST (@@1)))
+ (cmp @@0 @{ build_real (TREE_TYPE (@@1), dconst0); @})
+ /* x != NaN is always true, other ops are always false. */
+ (if (REAL_VALUE_ISNAN (TREE_REAL_CST (@@1))
+ && ! HONOR_SNANS (@@1))
+ @{ constant_boolean_node (cmp == NE_EXPR, type); @}))))
+@end smallexample
+
+which has the second @code{if} in the else operand of the first.
+The @code{switch} expression takes @code{if} expressions as
+operands (which may not have else clauses) and as a last operand
+a replacement expression which should be enabled by default if
+no other condition evaluated to true.
+
Captures can also be used for capturing results of sub-expressions.
@smallexample
