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Re: [patch][simplify-rtx] Fix 16-bit -> 64-bit multiply and accumulate
On 25/05/11 14:47, Joseph S. Myers wrote:
The shift must be by a positive constant amount, strictly less than the
precision (GET_MODE_PRECISION) of the mode (of the value being shifted).
If that applies, the relevant number of bits is the precision of the mode
minus the number of bits of the shift. For an extension, just take the
number of bits in the inner mode. Add the two numbers of bits; if the
result does not exceed the number of bits in the mode (of the operands and
the multiplication) then the multiplication won't overflow.
I believe the attached should implement what you describe.
Is the patch OK now?
Andrew
2011-05-26 Bernd Schmidt <bernds@codesourcery.com>
Andrew Stubbs <ams@codesourcery.com>
gcc/
* simplify-rtx.c (simplify_unary_operation_1): Canonicalize widening
multiplies.
* doc/md.texi (Canonicalization of Instructions): Document widening
multiply canonicalization.
gcc/testsuite/
* gcc.target/arm/mla-2.c: New test.
--- a/gcc/doc/md.texi
+++ b/gcc/doc/md.texi
@@ -5840,6 +5840,23 @@ Equality comparisons of a group of bits (usually a single bit) with zero
will be written using @code{zero_extract} rather than the equivalent
@code{and} or @code{sign_extract} operations.
+@cindex @code{mult}, canonicalization of
+@item
+@code{(sign_extend:@var{m1} (mult:@var{m2} (sign_extend:@var{m2} @var{x})
+(sign_extend:@var{m2} @var{y})))} is converted to @code{(mult:@var{m1}
+(sign_extend:@var{m1} @var{x}) (sign_extend:@var{m1} @var{y}))}, and likewise
+for @code{zero_extend}.
+
+@item
+@code{(sign_extend:@var{m1} (mult:@var{m2} (ashiftrt:@var{m2}
+@var{x} @var{s}) (sign_extend:@var{m2} @var{y})))} is converted
+to @code{(mult:@var{m1} (sign_extend:@var{m1} (ashiftrt:@var{m2}
+@var{x} @var{s})) (sign_extend:@var{m1} @var{y}))}, and likewise for
+patterns using @code{zero_extend} and @code{lshiftrt}. If the second
+operand of @code{mult} is also a shift, then that is extended also.
+This transformation is only applied when it can be proven that the
+original operation had sufficient precision to prevent overflow.
+
@end itemize
Further canonicalization rules are defined in the function
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -1000,6 +1000,48 @@ simplify_unary_operation_1 (enum rtx_code code, enum machine_mode mode, rtx op)
&& GET_CODE (XEXP (XEXP (op, 0), 1)) == LABEL_REF)
return XEXP (op, 0);
+ /* Extending a widening multiplication should be canonicalized to
+ a wider widening multiplication. */
+ if (GET_CODE (op) == MULT)
+ {
+ rtx lhs = XEXP (op, 0);
+ rtx rhs = XEXP (op, 1);
+ enum rtx_code lcode = GET_CODE (lhs);
+ enum rtx_code rcode = GET_CODE (rhs);
+
+ /* Widening multiplies usually extend both operands, but sometimes
+ they use a shift to extract a portion of a register. */
+ if ((lcode == SIGN_EXTEND
+ || (lcode == ASHIFTRT && CONST_INT_P (XEXP (lhs, 1))))
+ && (rcode == SIGN_EXTEND
+ || (rcode == ASHIFTRT && CONST_INT_P (XEXP (rhs, 1)))))
+ {
+ enum machine_mode lmode = GET_MODE (lhs);
+ enum machine_mode rmode = GET_MODE (rhs);
+ int bits;
+
+ if (lcode == ASHIFTRT)
+ /* Number of bits not shifted off the end. */
+ bits = GET_MODE_PRECISION (lmode) - INTVAL (XEXP (lhs, 1));
+ else /* lcode == SIGN_EXTEND */
+ /* Size of inner mode. */
+ bits = GET_MODE_PRECISION (GET_MODE (XEXP (lhs, 0)));
+
+ if (rcode == ASHIFTRT)
+ bits += GET_MODE_PRECISION (rmode) - INTVAL (XEXP (rhs, 1));
+ else /* rcode == SIGN_EXTEND */
+ bits += GET_MODE_PRECISION (GET_MODE (XEXP (rhs, 0)));
+
+ /* We can only widen multiplies if the result is mathematiclly
+ equivalent. I.e. if overflow was impossible. */
+ if (bits <= GET_MODE_PRECISION (GET_MODE (op)))
+ return simplify_gen_binary
+ (MULT, mode,
+ simplify_gen_unary (SIGN_EXTEND, mode, lhs, lmode),
+ simplify_gen_unary (SIGN_EXTEND, mode, rhs, rmode));
+ }
+ }
+
/* Check for a sign extension of a subreg of a promoted
variable, where the promotion is sign-extended, and the
target mode is the same as the variable's promotion. */
@@ -1071,6 +1113,48 @@ simplify_unary_operation_1 (enum rtx_code code, enum machine_mode mode, rtx op)
&& GET_MODE_SIZE (mode) <= GET_MODE_SIZE (GET_MODE (XEXP (op, 0))))
return rtl_hooks.gen_lowpart_no_emit (mode, op);
+ /* Extending a widening multiplication should be canonicalized to
+ a wider widening multiplication. */
+ if (GET_CODE (op) == MULT)
+ {
+ rtx lhs = XEXP (op, 0);
+ rtx rhs = XEXP (op, 1);
+ enum rtx_code lcode = GET_CODE (lhs);
+ enum rtx_code rcode = GET_CODE (rhs);
+
+ /* Widening multiplies usually extend both operands, but sometimes
+ they use a shift to extract a portion of a register. */
+ if ((lcode == ZERO_EXTEND
+ || (lcode == LSHIFTRT && CONST_INT_P (XEXP (lhs, 1))))
+ && (rcode == ZERO_EXTEND
+ || (rcode == LSHIFTRT && CONST_INT_P (XEXP (rhs, 1)))))
+ {
+ enum machine_mode lmode = GET_MODE (lhs);
+ enum machine_mode rmode = GET_MODE (rhs);
+ int bits;
+
+ if (lcode == LSHIFTRT)
+ /* Number of bits not shifted off the end. */
+ bits = GET_MODE_PRECISION (lmode) - INTVAL (XEXP (lhs, 1));
+ else /* lcode == ZERO_EXTEND */
+ /* Size of inner mode. */
+ bits = GET_MODE_PRECISION (GET_MODE (XEXP (lhs, 0)));
+
+ if (rcode == LSHIFTRT)
+ bits += GET_MODE_PRECISION (rmode) - INTVAL (XEXP (rhs, 1));
+ else /* rcode == ZERO_EXTEND */
+ bits += GET_MODE_PRECISION (GET_MODE (XEXP (rhs, 0)));
+
+ /* We can only widen multiplies if the result is mathematiclly
+ equivalent. I.e. if overflow was impossible. */
+ if (bits <= GET_MODE_PRECISION (GET_MODE (op)))
+ return simplify_gen_binary
+ (MULT, mode,
+ simplify_gen_unary (ZERO_EXTEND, mode, lhs, lmode),
+ simplify_gen_unary (ZERO_EXTEND, mode, rhs, rmode));
+ }
+ }
+
/* (zero_extend:M (zero_extend:N <X>)) is (zero_extend:M <X>). */
if (GET_CODE (op) == ZERO_EXTEND)
return simplify_gen_unary (ZERO_EXTEND, mode, XEXP (op, 0),
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/mla-2.c
@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -march=armv7-a" } */
+
+long long foolong (long long x, short *a, short *b)
+{
+ return x + *a * *b;
+}
+
+/* { dg-final { scan-assembler "smlalbb" } } */