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Re: [PATCH] fix c/39902 for decimal float constant folding


On Fri, Jun 26, 2009 at 7:33 PM, Janis Johnson<janis187@us.ibm.com> wrote:
> In decimal floating-point arithmetic, trailing zeroes are significant;
> the values 1.DD, 1.0DD, and 1.00DD are all different, and arithmetic on
> each of those values gives a different result. ?This means that GCC
> should not perform optimizations like replacing (x * 1) with x when
> the constant is a decimal float value.

Is this really true for real_zerop as well?  I do find it odd that zero doesn't
have a single exact representation in decimal floating-point.

> This patch modifies several functions that check for specific values of
> real constants so they don't handle decimal float values.
>
> Tested with bootstrap and regtest of all languages but Ada on
> powerpc64-linux for -m32/-m64. ?OK for trunk and, after testing there,
> on 4.4 and 4.3 branches? ?This is not a regression but it does fix
> wrong code.

If the real_zerop change is really warranted the patch is ok for trunk
and the branches.

Thanks,
Richard.


> 2009-06-26 ?Janis Johnson ?<janis187@us.ibm.com>
>
> gcc/
> ? ? ? ?PR c/39902
> ? ? ? ?* tree.c (real_zerop, real_onep, real_twop, real_minus_onep):
> ? ? ? ?Special-case decimal float constants.
>
> gcc/testsuite/
> ? ? ? ?PR c/39902
> ? ? ? ?* gcc.dg/dfp/pr39902.c: New test.
>
> Index: gcc/tree.c
> ===================================================================
> --- gcc/tree.c ?(revision 148858)
> +++ gcc/tree.c ?(working copy)
> @@ -1591,7 +1591,8 @@ tree_floor_log2 (const_tree expr)
> ? ? ? ? ?: floor_log2 (low));
> ?}
>
> -/* Return 1 if EXPR is the real constant zero. ?*/
> +/* Return 1 if EXPR is the real constant zero. ?Trailing zeroes matter for
> + ? decimal float constants, so don't return 1 for them. ?*/
>
> ?int
> ?real_zerop (const_tree expr)
> @@ -1599,13 +1600,16 @@ real_zerop (const_tree expr)
> ? STRIP_NOPS (expr);
>
> ? return ((TREE_CODE (expr) == REAL_CST
> - ? ? ? ? ?&& REAL_VALUES_EQUAL (TREE_REAL_CST (expr), dconst0))
> + ? ? ? ? ?&& REAL_VALUES_EQUAL (TREE_REAL_CST (expr), dconst0)
> + ? ? ? ? ?&& !(DECIMAL_FLOAT_MODE_P (TYPE_MODE (TREE_TYPE (expr)))))
> ? ? ? ? ?|| (TREE_CODE (expr) == COMPLEX_CST
> ? ? ? ? ? ? ?&& real_zerop (TREE_REALPART (expr))
> ? ? ? ? ? ? ?&& real_zerop (TREE_IMAGPART (expr))));
> ?}
>
> -/* Return 1 if EXPR is the real constant one in real or complex form. ?*/
> +/* Return 1 if EXPR is the real constant one in real or complex form.
> + ? Trailing zeroes matter for decimal float constants, so don't return
> + ? 1 for them. ?*/
>
> ?int
> ?real_onep (const_tree expr)
> @@ -1613,13 +1617,15 @@ real_onep (const_tree expr)
> ? STRIP_NOPS (expr);
>
> ? return ((TREE_CODE (expr) == REAL_CST
> - ? ? ? ? ?&& REAL_VALUES_EQUAL (TREE_REAL_CST (expr), dconst1))
> + ? ? ? ? ?&& REAL_VALUES_EQUAL (TREE_REAL_CST (expr), dconst1)
> + ? ? ? ? ?&& !(DECIMAL_FLOAT_MODE_P (TYPE_MODE (TREE_TYPE (expr)))))
> ? ? ? ? ?|| (TREE_CODE (expr) == COMPLEX_CST
> ? ? ? ? ? ? ?&& real_onep (TREE_REALPART (expr))
> ? ? ? ? ? ? ?&& real_zerop (TREE_IMAGPART (expr))));
> ?}
>
> -/* Return 1 if EXPR is the real constant two. ?*/
> +/* Return 1 if EXPR is the real constant two. ?Trailing zeroes matter
> + ? for decimal float constants, so don't return 1 for them. ?*/
>
> ?int
> ?real_twop (const_tree expr)
> @@ -1627,13 +1633,15 @@ real_twop (const_tree expr)
> ? STRIP_NOPS (expr);
>
> ? return ((TREE_CODE (expr) == REAL_CST
> - ? ? ? ? ?&& REAL_VALUES_EQUAL (TREE_REAL_CST (expr), dconst2))
> + ? ? ? ? ?&& REAL_VALUES_EQUAL (TREE_REAL_CST (expr), dconst2)
> + ? ? ? ? ?&& !(DECIMAL_FLOAT_MODE_P (TYPE_MODE (TREE_TYPE (expr)))))
> ? ? ? ? ?|| (TREE_CODE (expr) == COMPLEX_CST
> ? ? ? ? ? ? ?&& real_twop (TREE_REALPART (expr))
> ? ? ? ? ? ? ?&& real_zerop (TREE_IMAGPART (expr))));
> ?}
>
> -/* Return 1 if EXPR is the real constant minus one. ?*/
> +/* Return 1 if EXPR is the real constant minus one. ?Trailing zeroes
> + ? matter for decimal float constants, so don't return 1 for them. ?*/
>
> ?int
> ?real_minus_onep (const_tree expr)
> @@ -1641,7 +1649,8 @@ real_minus_onep (const_tree expr)
> ? STRIP_NOPS (expr);
>
> ? return ((TREE_CODE (expr) == REAL_CST
> - ? ? ? ? ?&& REAL_VALUES_EQUAL (TREE_REAL_CST (expr), dconstm1))
> + ? ? ? ? ?&& REAL_VALUES_EQUAL (TREE_REAL_CST (expr), dconstm1)
> + ? ? ? ? ?&& !(DECIMAL_FLOAT_MODE_P (TYPE_MODE (TREE_TYPE (expr)))))
> ? ? ? ? ?|| (TREE_CODE (expr) == COMPLEX_CST
> ? ? ? ? ? ? ?&& real_minus_onep (TREE_REALPART (expr))
> ? ? ? ? ? ? ?&& real_zerop (TREE_IMAGPART (expr))));
> Index: gcc/testsuite/gcc.dg/dfp/pr39902.c
> ===================================================================
> --- gcc/testsuite/gcc.dg/dfp/pr39902.c ?(revision 0)
> +++ gcc/testsuite/gcc.dg/dfp/pr39902.c ?(revision 0)
> @@ -0,0 +1,239 @@
> +/* { dg-options "--std=gnu99" } */
> +
> +/* Check that optimizations like (x * 1) to x, or (x * -1) to -x,
> + ? do not apply to decimal float computations where trailing zeroes
> + ? are significant. ?*/
> +
> +extern void abort (void);
> +int failcnt;
> +
> +#ifdef DBG
> +extern int printf (const char *, ...);
> +#define FAILURE { printf ("failed at line %d\n", __LINE__); failcnt++; }
> +#else
> +#define FAILURE abort ();
> +#endif
> +
> +#define COMPARE32(A,B) \
> + ?A.i == B.i
> +
> +#define COMPARE64(A,B) \
> + ?A.i[0] == B.i[0] && A.i[1] == B.i[1]
> +
> +#define COMPARE128(A,B) \
> + ?A.i[0] == B.i[0] && A.i[1] == B.i[1] && A.i[2] == B.i[2] && A.i[3] == B.i[3]
> +
> +typedef union {
> + ?_Decimal32 d;
> + ?unsigned int i;
> +} u32;
> +
> +typedef union {
> + ?_Decimal64 d;
> + ?unsigned int i[2];
> +} u64;
> +
> +typedef union {
> + ?_Decimal128 d;
> + ?unsigned int i[4];
> +} u128;
> +
> +volatile u32 p32_1;
> +volatile u32 p32_1_0;
> +volatile u32 p32_2_0;
> +volatile u32 m32_1;
> +volatile u32 m32_1_0;
> +volatile u32 m32_2_0;
> +volatile u32 a32;
> +
> +volatile u64 p64_1;
> +volatile u64 p64_1_0;
> +volatile u64 p64_2_0;
> +volatile u64 m64_1;
> +volatile u64 m64_1_0;
> +volatile u64 m64_2_0;
> +volatile u64 a64;
> +
> +volatile u128 p128_1;
> +volatile u128 p128_1_0;
> +volatile u128 p128_2_0;
> +volatile u128 m128_1;
> +volatile u128 m128_1_0;
> +volatile u128 m128_2_0;
> +volatile u128 a128;
> +
> +void
> +init32 (void)
> +{
> + ?p32_1.d = 1.DF;
> + ?p32_1_0.d = 1.0DF;
> + ?p32_2_0.d = 2.0DF;
> + ?m32_1.d = -1.DF;
> + ?m32_1_0.d = -1.0DF;
> + ?m32_2_0.d = -2.0DF;
> +}
> +
> +void
> +init64 (void)
> +{
> + ?p64_1.d = 1.DD;
> + ?p64_1_0.d = 1.0DD;
> + ?p64_2_0.d = 2.0DD;
> + ?m64_1.d = -1.DD;
> + ?m64_1_0.d = -1.0DD;
> + ?m64_2_0.d = -2.0DD;
> +}
> +
> +void
> +init128 (void)
> +{
> + ?p128_1.d = 1.DL;
> + ?p128_1_0.d = 1.0DL;
> + ?p128_2_0.d = 2.0DL;
> + ?m128_1.d = -1.DL;
> + ?m128_1_0.d = -1.0DL;
> + ?m128_2_0.d = -2.0DL;
> +}
> +
> +void
> +doit32 (void)
> +{
> + ?/* Multiplying by a value with no trailing zero should not change the
> + ? ? quantum exponent. ?*/
> +
> + ?a32.d = p32_2_0.d * p32_1.d;
> + ?if (! (COMPARE32 (a32, p32_2_0)))
> + ? ?FAILURE
> +
> + ?a32.d = p32_2_0.d * 1.DF;
> + ?if (! (COMPARE32 (a32, p32_2_0)))
> + ? ?FAILURE
> +
> + ?a32.d = p32_2_0.d * m32_1.d;
> + ?if (! (COMPARE32 (a32, m32_2_0)))
> + ? ?FAILURE
> +
> + ?a32.d = p32_2_0.d * -1.DF;
> + ?if (! (COMPARE32 (a32, m32_2_0)))
> + ? ?FAILURE
> +
> + ?/* Multiplying by a value with a trailing zero should change the
> + ? ? quantum exponent. ?*/
> +
> + ?a32.d = p32_2_0.d * p32_1_0.d;
> + ?if (COMPARE32 (a32, p32_2_0))
> + ? ?FAILURE
> +
> + ?a32.d = p32_2_0.d * 1.0DF;
> + ?if (COMPARE32 (a32, p32_2_0))
> + ? ?FAILURE
> +
> + ?a32.d = p32_2_0.d * m32_1_0.d;
> + ?if (COMPARE32 (a32, m32_2_0))
> + ? ?FAILURE
> +
> + ?a32.d = p32_2_0.d * -1.0DF;
> + ?if (COMPARE32 (a32, m32_2_0))
> + ? ?FAILURE
> +}
> +
> +void
> +doit64 (void)
> +{
> + ?/* Multiplying by a value with no trailing zero should not change the
> + ? ? quantum exponent. ?*/
> +
> + ?a64.d = p64_2_0.d * p64_1.d;
> + ?if (! (COMPARE64 (a64, p64_2_0)))
> + ? ?FAILURE
> +
> + ?a64.d = p64_2_0.d * 1.DD;
> + ?if (! (COMPARE64 (a64, p64_2_0)))
> + ? ?FAILURE
> +
> + ?a64.d = p64_2_0.d * m64_1.d;
> + ?if (! (COMPARE64 (a64, m64_2_0)))
> + ? ?FAILURE
> +
> + ?a64.d = p64_2_0.d * -1.DD;
> + ?if (! (COMPARE64 (a64, m64_2_0)))
> + ? ?FAILURE
> +
> + ?/* Multiplying by a value with a trailing zero should change the
> + ? ? quantum exponent. ?*/
> +
> + ?a64.d = p64_2_0.d * p64_1_0.d;
> + ?if (COMPARE64 (a64, p64_2_0))
> + ? ?FAILURE
> +
> + ?a64.d = p64_2_0.d * 1.0DD;
> + ?if (COMPARE64 (a64, p64_2_0))
> + ? ?FAILURE
> +
> + ?a64.d = p64_2_0.d * m64_1_0.d;
> + ?if (COMPARE64 (a64, m64_2_0))
> + ? ?FAILURE
> +
> + ?a64.d = p64_2_0.d * -1.0DD;
> + ?if (COMPARE64 (a64, m64_2_0))
> + ? ?FAILURE
> +}
> +
> +void
> +doit128 (void)
> +{
> + ?/* Multiplying by a value with no trailing zero should not change the
> + ? ? quantum exponent. ?*/
> +
> + ?a128.d = p128_2_0.d * p128_1_0.d;
> + ?if (COMPARE128 (a128, p128_2_0))
> + ? ?FAILURE
> +
> + ?a128.d = p128_2_0.d * 1.0DD;
> + ?if (COMPARE128 (a128, p128_2_0))
> + ? ?FAILURE
> +
> + ?a128.d = p128_2_0.d * m128_1_0.d;
> + ?if (COMPARE128 (a128, m128_2_0))
> + ? ?FAILURE
> +
> + ?a128.d = p128_2_0.d * -1.0DD;
> + ?if (COMPARE128 (a128, m128_2_0))
> + ? ?FAILURE
> +
> + ?/* Multiplying by a value with a trailing zero should change the
> + ? ? quantum exponent. ?*/
> +
> + ?a128.d = p128_2_0.d * p128_1.d;
> + ?if (! (COMPARE128 (a128, p128_2_0)))
> + ? ?FAILURE
> +
> + ?a128.d = p128_2_0.d * 1.DD;
> + ?if (! (COMPARE128 (a128, p128_2_0)))
> + ? ?FAILURE
> +
> + ?a128.d = p128_2_0.d * m128_1.d;
> + ?if (! (COMPARE128 (a128, m128_2_0)))
> + ? ?FAILURE
> +
> + ?a128.d = p128_2_0.d * -1.DD;
> + ?if (! (COMPARE128 (a128, m128_2_0)))
> + ? ?FAILURE
> +}
> +
> +int
> +main (void)
> +{
> + ?init32 ();
> + ?init64 ();
> + ?init128 ();
> +
> + ?doit32 ();
> + ?doit64 ();
> + ?doit128 ();
> +
> + ?if (failcnt != 0)
> + ? ?abort ();
> +
> + ?return 0;
> +}
>
>
>


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