See: http://gcc.gnu.org/ml/gcc-patches/2011-05/msg02150.html
for an analysis. OK for trunk? Thanks. H.J. --- 2011-05-21 H.J. Lu <hongjiu...@intel.com> PR rtl-optimization/49088 * combine.c (force_to_mode): If X is narrower than MODE and we want all the bits in X's mode, just use the operand when it is CONST_INT. diff --git a/gcc/combine.c b/gcc/combine.c index 8af86f2..710fe0e 100644 --- a/gcc/combine.c +++ b/gcc/combine.c @@ -8321,8 +8321,26 @@ force_to_mode (rtx x, enum machine_mode mode, unsigned HOST_WIDE_INT mask, /* For most binary operations, just propagate into the operation and change the mode if we have an operation of that mode. */ - op0 = force_to_mode (XEXP (x, 0), mode, mask, next_select); - op1 = force_to_mode (XEXP (x, 1), mode, mask, next_select); + /* If X is narrower than MODE and we want all the bits in X's + mode, just use the operand when it is CONST_INT. */ + if (SCALAR_INT_MODE_P (mode) + && GET_MODE_SIZE (GET_MODE (x)) < GET_MODE_SIZE (mode) + && (GET_MODE_MASK (GET_MODE (x)) & ~mask) == 0) + { + if (CONST_INT_P (XEXP (x, 0))) + op0 = XEXP (x, 0); + else + op0 = force_to_mode (XEXP (x, 0), mode, mask, next_select); + if (CONST_INT_P (XEXP (x, 1))) + op1 = XEXP (x, 1); + else + op1 = force_to_mode (XEXP (x, 1), mode, mask, next_select); + } + else + { + op0 = force_to_mode (XEXP (x, 0), mode, mask, next_select); + op1 = force_to_mode (XEXP (x, 1), mode, mask, next_select); + } /* If we ended up truncating both operands, truncate the result of the operation instead. */