Predicated execution is an effective technique for dealing with conditional branches in application programs. However, there are several problems associated with conventional compiler support for predicated execution. First, all paths of control are combined into a single path regardless of their execution frequency and size with conventional if-conversion techniques. Second, speculative execution is difficult to combine with predicated execution. In this paper, we propose the use of a new structure, referred to as the hyperblock, to overcome these problems. The hyperblock is an efficient structure to utilize predicated execution for both compiletime optimization and scheduling. Preliminary experimental results show that the hyperblock is highly effective for a wide range of superscalar and VLIW processors.
Lamellar single crystals grown in dilute solutions can be used as templates for tethered chain analysis. Two series of diblock copolymers, poly(ethylene oxide)-block-polystyrene (PEO-b-PS) and poly(Llactic acid)-block-polystyrene (PLLA-b-PS), were used as model templates to generate tethered PS blocks on the single-crystal basal surfaces. Controlled and tunable reduced tethering density, σ ˜, defined by σπR g 2 (where σ is the tethered chain density and is equal to the reciprocal of the covered area of the chain and R g is the radius of gyration of this tethered chain in its end-free state at the same conditions), could be achieved in a broad range (up to 24) by changing the molecular weights (MW's) of the crystalline and amorphous blocks and by varying the crystallization temperature (T x ) of different PEO-b-PS and PLLA-b-PS solutions. For PEO and PLLA homopolymers crystallized in dilute solutions, the lamellar crystal thicknesses (d CRYST ) were observed to be proportional to the reciprocal undercooling ∆T (where ∆T ) T d -T x and T d is the equilibrium dissolution temperature of the crystals). The σ ˜of the tethered PS chains on the crystal surface increased with decreasing ∆T because at a fixed MW of the PEO or PLLA block, an increase in the d CRYST was evidence of a decrease in the number of folds. When we plotted the relationships between 1/d CRYST and T x for these two series of diblock copolymers, sudden and discontinuous changes of the slopes in some of these were observed at σ ˜) 3.7 (σ ˜*). This was as a result of the drastic interaction change of the neighboring PS tethered chains. An average reduced surface free energy of the tethered PS chains (Γ PS ) was defined and used as a parameter to characterize the PS tethered chain interactions. The relationship between Γ PS and σ ˜showed a discontinuous transition at σ ˜*, which had a close similarity to the hard-sphere-like interaction model. This could be identified as the onset of the tethered PS chain overcrowding in solution. This transition indicates that the extra entropic surface free energy created by the repulsion of tethered PS chains started to affect the nucleation barrier of the PEO or PLLA block crystallization. On the basis of the scaling laws, the onset of highly stretched brush regime could be identified at σ ˜) 14.3 (σ ˜**). In the Γ PS vs σ ˜plot, the transition appears to be continuous. Thus, a crossover regime in the tethered PS chains exists between σ ˜* ) 3.7 and σ ˜** ) 14.3. It is defined as the regime where the interaction of the tethered PS chains undergoes changes from being noninteracting toward penetration to, finally, chain stretching normal to the surface.
We proposed an approach to precisely control the density of tethered chains on solid substrates using PEO-b-PS and PLLA-b-PS. As the crystallization temperature Tx increased, the PEO or PLLA lamellar crystal thickness d(L) increased as well as the reduced tethering density sigma; of the PS chains. The onset of tethered PS chains overcrowding in solution occurs at sigma(*) approximately 3.7-3.8 as evidenced by an abrupt change in the slope between (d(L))(-1) and Tx. This results from the extra surface free energy created by the tethered chain that starts to affect the growth barrier of the crystalline blocks.
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